Grease and flour a 13-by-9-by-2-inch baking pan. In a medium bowl, combine flour, baking powder and salt; set aside. In large bowl of electric mixer on medium speed, beat peanut butter, butter and brown sugar until light and fluffy. Add eggs, one at a time, beating after each addition. Add milk and vanilla; beat until well mixed. Add flour mixture; beat until well mixed. Pour batter into prepared pan. Bake in a preheated 350-degree oven 35 to 40 minutes, or until cake springs back when lightly touched. Let cake cool in pan on wire rack for 10 minutes; remove cake from pan and let cool completely.

Cut cake in half horizontally with a long sharp knife or by marking the sides of the cake with toothpicks and carefully drawing a long piece of thread or dental floss through the cake. Using long metal spatulas, carefully lift off the top layer. Spread bottom layer with grapejelly. Replace top layer. Frost cake with peanut butter frosting.

Frosting: In large bowl of electric mixer on medium speed, beat peanut butter and butter until light and fluffy. Gradually beat in confectioners' sugar alternately with milk until frosting is a good spreading consistency. Beat in vanilla. Yield: Enough frosting for one 13-by-9-by-2-inch cake.