Preheat oven to 350°F. Combine flour, baking powder, soda and salt. In large bowl, cream butter and sugar with electric mixer until light and fluffy. Add eggs, one at a time, and vanilla; beat thoroughly. Add flour mixture to creamed mixture alternately with milk, beating well after each addition. Add melted chocolate and beat thoroughly. Pour into prepared pan. Bake 20-25 minutes or until toothpick inserted comes out clean. Cool 10 minutes in pan on rack; remove and cool completely before decorating.