Heat oven to 350°F. Grease and lightly flour 13x9-inch pan. In medium bowl, stir flour, baking powder and salt until well blended.

In large bowl, beat granulated sugar and 3/4 cup softened butter with electric mixer on medium speed until light and fluffy, scraping bowl occasionally. Beat in 1 1/2 teaspoons vanilla and eggs until well blended. Alternately add flour mixture and 1 1/4 cups milk, beating well and scraping bowl after each addition. Pour batter evenly into pan.

Bake 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.

Meanwhile, in small microwavable bowl, microwave chocolate uncovered on High 30 to 60 seconds, stirring every 30 seconds, until melted and smooth. Cool 10 minutes.

In large bowl, beat remaining Frosting ingredients with electric mixer on low speed until creamy. Gradually beat in melted chocolate until well blended. Immediately spread on cake.