In a mixer, cream together 170g softened butter, 300g sugar (1 1/2 cups), and 2 tsp vanilla extract. Beat in 3 eggs. Set aside.
In a large bowl, mix together 100g flour (3/4 cup), with 60g cocoa (1/2 cup), 1/2 tsp salt, and 1/2 tsp baking powder. Fold the wet mixture into the flour mixture and fold lightly till blended.
Bake in a 20cm (8inch) square pan, lined with parchment paper for 25 minutes at 170c till a knife comes out clean. Do not overbake!
Let cool before removing from the pan and cutting into 16 squares.
Optional: for an extra chocolatey flavour fold in 50g chopped dark chocolate into the mixture before pouring into the pan,