Preheat the oven to 350F. Lightly grease and flour a shallow 8" square cake pan.

Place the butter, sugars, dark chocolate, white chocolate, and light corn syrup (or golden syrup) in a heavy-based saucepan and heat gently, stirring until the mixture is well blended and smooth.  Remove from heat and let cool.

Beat together the eggs and extract.  Beat in the cooled chocolate mixture.

Sift together the flour, unsweetened cocoa, and baking powder, and fold carefully into egg and chocolate mixture, using a metal spoon or a spatula.

Spoon mixture into the prepared pan and bake for 25 minutes, until the top is crisp and the edge of the cake is beginning to shrink away from the pan.  The inside of the cake will still be quite dense and soft to the touch.

Allow the cake to cool completely in the pan, then cut it into squares to serve.