Preheat oven to 350 degrees.
In mixer, cream butter and both sugars until fluffy. Add melted chocolate and yogurt mixing well.
Add eggs one at a time, mix for 1 minute after each egg addition.
Add vanilla and baking powder, mixing well. Stir in flour and any optional ingredients.
Spread into greased 13x9x2-inch baking pan and bake for 45-50 minutes.
Cool completely, cut, and serve.