Preheat oven to 325°F. Spray an 8x8-inch baking pan with non-stick vegetable spray. In a large saucepan, melt butter and chocolate, stirring until smooth; set aside to cool. Stir in eggs, flour, and pecans; mix thoroughly. Bake for 35 minutes. Cool in pan on a wire rack. Poke with a fork; drizzle amaretto over the top. Refrigerate overnight. Frost with favorite chocolate frosting, if desired.