200g dark chocolate, chopped to small chunks
125g unsalted butter
1/4 tsp salt
1 tsp vanilla extract
3 large eggs
175g caster sugar
125g plain flour
1 1/2 tsp baking powder
100g white chocolate, chopped to small chunks
Preheat the oven to 180°C/fan 160°C/gas mark 4. Grease and line a 15x23cm (or thereabouts) rectangular tin.
In a heatproof bowl suspended over a pan of simmering water (make sure that the base of the bowl does not touch the water), melt 150g of the dark chocolate. Stir in the butter, continuing to heat gently just until the butter has completely melted. Add the salt and vanilla and set aside to cool slightly.
In a large bowl, whisk the eggs and sugar together for 5 minutes (don’t cut this short!). Whisking the eggs helps to give these brownies a thin, crisp crust as they bake. During this time they’ll become thick, creamy and foamy. In a separate bowl, mix the flour and baking powder.
Now to combine everything: gently fold the chocolate mixture into the beaten eggs, stirring lightly until just combined. Add the flour mixture and mix. Add the remaining dark chocolate and the white chocolate.
Pour into the prepared tin and bake for 30-40 minutes. The exact cooking time will depends on the dimensions of your tin, but just check from 25 minutes onwards by inserting a small knife or toothpick into the centre of the brownie – if it comes out a little sticky, but not covered in batter, the brownie is ready.