Preheat the oven to 180C/350F/Gas 4.
Prepare the cake tin. Grease an 18cm/7in square cake tin and line with non-stick baking parchment.
In a clean bowl, cream together the butter and sugar until pale and fluffy. Beat in the eggs, one at a time.
Chop the chocolate and put to one side.
Chop the pecans, if using. Gently fold the nuts and chocolate into the butter and egg mixture. Add the vanilla extract.
Dissolve the coffee granules in two tablespoons of warm water and add to the mixture. Mix together well.
Add the baking powder, then sift in the flour and cocoa powder. Mix well; the mixture will be quite wet.
Spoon the brownie batter into the prepared tin and level out the top with a spatula.
Bake in the oven for 35-40 minutes. Allow to cool in the tin for ten minutes, then place on a wire rack to cool.
To serve, cut into rectangles and dust with icing sugar.