step 1
Preheat oven 160C/Gas 3/fan oven 140C. Butter and line the base of a shallow 23cm/9in square cake tin. Melt the chocolate and butter together, stir well and cool.
step 2
Whisk the eggs until pale, then whisk in the sugar until thick and glossy and well combined.
step 3
Gently fold in the melted chocolate mixture, then sift in the flour and baking powder and stir gently until smooth.
step 4
Pour into the prepared tin and bake for 30-35 minutes, or until firm to the touch. Test by inserting a wooden cocktail stick into the middle-there should be a few moist crumbs sticking to it. The mixture will still be soft in the centre but will firm up on cooling.
step 5
Cool in the tin on a wire rack for at least 1 hour, then cut into 16 squares and finish cooling on the rack, although I must admit it is much better to skip all this cooling malarky and eat them immediately when warm and fudgy!