Cake: Combine first 4 ingredients in a saucepan; cook over medium heat, stirring constantly, until smooth.

Combine flour, soda and sugar in a large mixing bowl. Add eggs, milk, and vanilla; beat at low speed of an electric mixer until blended. Add peanut butter mixture; mix well. Pour batter into a greased and floured 13x9x2-inch pan.

Frosting: Combine first 3 ingredients in a medium saucepan; cook over medium heat, stirring constantly, until smooth. Remove from heat. Add powdered sugar and vanilla, stirring until mixture is spreading consistency.

Yields: 2 1/2 cups