Grease a 20 cm (8 inch) square tin and line the base with nonstick baking paper.
Melt the butter and chocolate together in a saucepan over a low heat. Whisk the eggs and sugar together in a bowl until the mixture is pale and creamy. Stir the melted chocolate mixture into the egg mixture. Sift in the flour and baking powder and fold together. Turn into the tin and level the surface.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 25 minutes until the brownies are firm on top and a skewer inserted into the centre comes out clean. Cool in the tin for 5 minutes, then cut into squares.