Preheat oven to 350 degrees. Grease and flour 2 9" cake pans. In large mixing bowl, cream butter and peanut butter until light and fluffy. Add brown sugar. Mix to blend. Add eggs, one at a time, mixing well after each addition. In small bowl, combine flour, baking powder and salt. Add flour mixture to butter and peanut butter mixture alternately with milk, beginning and ending with flour mixture. Add vanilla. Pour batter into pans. Bake until cake tests done, about 45 minutes. Cool on wire rack to room temperature before frosting the cake. Spread half of Peanut Butter Filling over tops of each cake. Chill. Spread half of warm Chocolate Glaze over peanut butter topping on each cake, using metal spatula dipped in hot water. As glaze cools, it will thicken. Peanut Butter Filling: Cream ingredients together until light and fluffy. Chocolate Glaze: Place water and butter in small saucepan. Bring to boil. Add cocoa, sugar and vanilla to water mixture. Mix until smooth. NOTE: This recipe will make two 9 inch cakes or one stacked layer cake. If making the stacked layer cake, the Peanut Butter Filling should be put between the layers and on top of the cake before topping it with the Chocolate Glaze.