Preheat oven to 350°F. Spray pans with vegetable pan spray. In large bowl, cream butter and sugar with electric mixer until light and fluffy. Add eggs, one at a time, and vanilla; mix well. Combine flour, baking powder and salt. Add to butter mixture alternately with milk; mix well after each addition. Continue beating 1 minute. Divide batter in half. Stir melted chocolate into half the batter; mix well.