Heat the oven to 180C/fan 160C/gas 4. Line a 22cm square brownie tin with baking parchment. Melt the butter and chocolate together in a microwave or in a bowl set over a pan of simmering water. Cool to room temperature. Whisk the eggs and sugar together until the mixture is light and fluffy.
Fold the chocolate mixture into the egg mixture and sift on the flour, baking powder and cocoa. Fold this in to give a fudgy batter. Bake for 25-30 minutes or until the top is cracked but the middle just set. Cool completely, then lift out of the tin and cut.