Melt butter on a low heat and add cocoa. Mix together. Add 4 eggs and mix together. Add sugar and vanilla extract. Chop up the chocolate and add to the wet mix. Sieve in the flour, salt, and baking powder, fold into the wet mixture.
Place in a square baking tin 21 cm x 21 cm with high sides.
Bake at 180°C for approx. 40-50 mins. For a crusty top leave in the oven for an extra ten minutes with the heat turned off.
Garnish options: Crystallised ginger or extra chopped chocolate.