Butter a 9-inch-square cake pan, line with parchment, then butter again and dust with flour. Make the cake mix and fill the pan three-quarters full (you'll have batter left over). Bake at 350 degrees F until a toothpick comes out clean, 30 to 35 minutes.
Let the cake cool in the pan 10 minutes, then turn out onto a rack and let cool completely. Transfer the cake to a work surface. Trim off the top using a serrated knife, then slice the cake in half horizontally.
Beat the butter and peanut butter with a mixer until smooth. Alternate adding the confectioners' sugar and milk, beating after each addition, until the frosting is fluffy and smooth. In another bowl, whisk the grape jelly until it is loose and spreadable.
Spread a thick layer of peanut butter frosting on the bottom cake half.
Spread the grape jelly on top of the peanut butter frosting, making sure to reach the edges. Top with the remaining cake layer.
Cut the cake in half diagonally and transfer to a serving plate. Separate the halves slightly to show the filling.