Preheat the oven to 180oC/350oF/Gas
4. Grease the base and sides of a 20cm square baking tin.
In a heatproof bowl set over a pan of
barely simmering water, melt the chocolate and butter or margarine.
Remove the bowl from the heat.
Add the sugar, flour, baking powder,
eggs, vanilla and half of the hazelnuts to the melted mixture and
stir well with a wooden spoon.
Pour the mixture into the prepared
tin. Bake in the oven for 10 minutes, then sprinkle the reserved
hazelnuts over the top. Return to the oven and continue baking until
firm to the touch, about 25 minutes.
Cool in the tin, set on a wire rack
for 10 minutes, then unmould on to the rack and cool completely.