Heat oven to 350°F. Grease 2 (9-inch) square baking pans. Prepare and bake cake mix according to package directions for 2 (8-inch) baking pans. Cool completely.
In medium bowl, beat peanut butter, butter, powdered sugar and 1 tablespoon milk with an electric mixer until light and fluffy; adding more milk if necessary.
With serrated knife, carefully trim tops of cake layers to remove browned tops and level cake layers. Place one layer cut side up on serving plate. Spread peanut butter mixture over cut side of cake layer and top with jam. Top with second cake layer, cut side up.
Slice cake diagonally to resemble a sandwich.