Preheat the oven to 350 degrees F. Grease
an 8-by-8-by-2-inch pan.
Melt the chocolate and shortening together in a small saucepan. Stir the sugar into the well-beaten eggs in a large bowl. Combine the chocolate and the egg mixtures in the bowl.
Add the baking powder and salt to flour and sift. Add the flour mixture to the chocolate mixture; add the vanilla and nuts, if using. Stir to combine.
Spread the dough evenly in the pan. Bake 25 to 30 minutes. Cool 30 minutes, cut into squares and enjoy.