Preheat the oven to 180 C / Gas 4. Grease your size choice of pan.
Place the flour, butter, sugar, baking powder, xantham gum, eggs and cocoa powder in a bowl and mix. When these ingredients are all mixed in, break the chocolate into squares and add them with the nuts into the mixture. Pour the mix into the pan evenly.
Bake for 30-35 minutes. To check to see if they are done, stick a skewer into the middle of the pan. If it come out clean they are done; if it comes out with batter on, then leave in for a few more minutes. Leave to cool, then cut into appropriate sized pieces. :) Enjoy.