225 gm butter 140 gm cooking chocolate 5 eggs 1 3/4 cups (packed) light brown sugar 1 cup plain flour 1 1/2 teasp vanilla Optional: chopped nuts or 1 tbls instant coffee or 1 teasp grated lemon or orange rind or 1/2 teasp allspice or cinnamon or a mashed ripe banana or any or none of the above!
Butter a 22cm by 33 cm. pan. Preheat the oven to 180°C.
Soften the butter (but don't melt it). Melt the chocolate and let it cool a bit. Cream the butter with the sugar and eggs. Add vanilla, then beat in the melted, cooled chocolate and the flour.
Spread into pan and bake 20-30 minutes in a moderate oven.
Yet another option. Instead of uniformly blending in the chocolate you can marble it by drizzling it in at the end and only partly blending it. This is what I did, but I didn't really like the effect - the chocolate tended to set into lumps rather than blend at all.