In a mixer, cream together 170g softened butter, 300g sugar (1 ½ cups), and 2 tsp vanilla extract. Beat in 3 eggs. Set aside.

In a large bowl, mix together 100g flour (¾ cup), with 60g cocoa (½ cup), ½ tsp salt, and ½ 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,