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Every Kiwi household has a version of this slice, and every version is slightly wrong according to someone’s mother. It’s the cheapest tray of baking you can make — a box of Weet-Bix, a bit of flour, some coconut and cocoa — and it’s been going into lunchboxes since long before anyone thought to photograph it.
This one makes 24 solid pieces in about 35 minutes, and it holds together properly, which is the part most recipes get wrong.
Why Weet-Bix, and not just more flour
Weet-Bix isn’t filler. It’s wheat that’s already been cooked, rolled and malted, so it brings three things flour can’t.
It stays slightly crunchy. Because the wheat is pre-cooked, it doesn’t dissolve into the butter and sugar the way raw flour does — you get flecks of texture right through the slice instead of a uniform crumb.
It adds malt. That faint toasty background flavour people can never quite identify in a good Weet-Bix slice is the malt extract in the biscuits. Cocoa alone won’t give you it.
And it soaks up butter. This is the one that matters for texture, and it’s why the ratio here is deliberate: three biscuits to one cup of flour. Push the Weet-Bix higher and the slice turns dry and sandy, because there’s more dry matter than the butter can carry.
How many Weet-Bix, exactly
Recipes are vague about this and it’s the commonest reason a slice fails. A standard NZ Weet-Bix biscuit is about 15g, so three biscuits is roughly 45g, which crushes down to about one loosely packed cup. If you’re crushing straight into a measuring cup, aim for 45–50g on the scales rather than trusting the cup — crushed fine, the same three biscuits pack into far less than a cup, and you’ll end up short.
Crush them in a bowl with the back of a spoon. A food processor turns them to powder, and powder is exactly what you don’t want.
Getting the base right
Two things decide whether this works.
Press it firmly. Not gently, not “level it off” — press it hard with the back of a spoon or the base of a glass until the surface is compacted and even. A loosely pressed base is the reason slices crumble when you cut them.
Ice it while it’s still warm. Not hot, not cold — warm, about ten minutes out of the oven. Warm base melts the icing just enough to bond it to the surface. Ice a cold slice and the icing sits on top as a separate layer that lifts off in one sheet the first time someone bites into it.
Frequently asked questions
How many Weet-Bix in a Weet-Bix slice?
Three, which is about 45 g. Weigh rather than measure by cup if you can — finely crushed Weet-Bix packs down to much less than a cup, and using a full cup of crushed biscuits makes the slice dry.
Can I use Weetabix instead?
Yes, but they’re not identical. UK Weetabix biscuits are larger (around 19 g each) and slightly less malty, so use two and a half rather than three, and expect a marginally plainer flavour.
Why is my Weet-Bix slice crumbly?
Almost always one of two things. Either the base wasn’t pressed firmly enough before baking, or there was too much Weet-Bix relative to the butter. If you crushed the biscuits to powder and measured a full cup, you’ve likely used close to double the intended amount.
Do I have to ice it?
No, and plenty of people don’t. Without icing it’s a plainer, more biscuit-like slice that suits a lunchbox better because it doesn’t melt. If you skip the icing, add an extra tablespoon of sugar to the base.
How long does it keep?
Five days in an airtight container at room temperature. It doesn’t need refrigerating, and the fridge dries it out.
Can I freeze it?
Yes. Freeze it iced, in a single layer with baking paper between the pieces, for up to three months. It thaws in about half an hour and goes straight into a lunchbox frozen if you’d rather.
Is it gluten free?
No. Weet-Bix is made from wheat, so the slice contains gluten from both the biscuits and the flour. Sanitarium’s gluten-free range doesn’t include a direct Weet-Bix substitute that behaves the same way in baking.
Variations
- Chocolate chip — stir 1/2 cup of chocolate chips through the dry mixture before adding the butter.
- Golden syrup — replace 2 tablespoons of the sugar with 2 tablespoons of golden syrup, warmed with the butter. Gives a chewier, denser slice.
- Sultana — add 1/2 cup of sultanas. Common in older NZ versions, and it works better than it sounds against the cocoa.
- No cocoa — leave the cocoa out of the base and ice it with plain lemon icing instead. This is closer to the original 1970s version.
| Prep Time | 15 minutes |
| Cook Time | 20 minutes |
| Servings |
pieces
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- 125 g butter melted
- 3/4 cup caster sugar 165 g
- 1 cup plain flour 150 g
- 3 Weet-Bix crushed, about 45 g
- 1/2 cup desiccated coconut 45 g
- 2 Tbsp cocoa powder
- 1 tsp baking powder
- 1 1/2 cups icing sugar 180 g
- 2 Tbsp cocoa powder
- 25 g butter softened
- 2-3 Tbsp boiling water
- 2 Tbsp desiccated coconut to finish
Ingredients
Base
Chocolate icing
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- Heat the oven to 180°C (160°C fan). Line a 20 × 30 cm slice tin with baking paper, leaving an overhang on the long sides to lift the slice out later.
- Crush the Weet-Bix in a bowl with the back of a spoon until broken into rough crumbs, not powder. You want some pieces the size of rolled oats.
- Add the flour, coconut, cocoa, baking powder and sugar to the same bowl and stir to combine.
- Pour in the melted butter and mix until every bit of the dry mixture is damp. It will look like coarse crumbs rather than a batter — that's correct.
- Tip into the tin and press down firmly and evenly with the back of a spoon until the surface is compacted. Take your time here; this is what holds the slice together.
- Bake for 20 minutes, until the surface is dry to the touch and the edges have just begun to pull away from the paper. It will still feel soft in the middle — it firms as it cools.
- Make the icing while it bakes. Sift the icing sugar and cocoa into a bowl, add the softened butter, then add the boiling water a tablespoon at a time, beating until smooth and just pourable.
- Leave the slice in the tin for 10 minutes, then spread the icing over the warm base and scatter with coconut.
- Cool completely in the tin before lifting out and cutting into 24 pieces. Cutting warm will drag the icing.