Heat oven to 350°F Combine flour, baking powder and salt in medium bowl. Set aside.
Beat sugar and 1 cup butter in large bowl at medium speed, scraping bowl occasionally, until creamy. Add eggs, two at a time, beating well after each addition. Add 2 teaspoons vanilla. Reduce speed to low; add flour mixture alternately with 1 cup milk, beating well after each addition.
Pour batter into greased 13x9-inch baking pan. Bake for 45 to 55 minutes or until toothpick inserted in center comes out clean. Cool completely.
Meanwhile, beat 1/2 cup butter and cooled chocolate in large bowl at medium speed until creamy. Gradually add powdered sugar, alternately with 1/4 cup milk and 2 teaspoons vanilla and scraping bowl occasionally, until well mixed. Frost cooled cake. Sprinkle with decorator sprinkles, if desired.
TIP: Add 1 to 2 tablespoons additional milk to frosting for desired spreading consistency.
TIP: This cake is very versatile. Frost the cake or, leave unfrosted and top each serving with sliced strawberries and whipped cream.