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Preheat oven to 350F. Spray square cake pan and line with parchment paper, then spray again. Make the cake mix and fill the pan half full. Bake until a toothpick comes out clean, about 30 to 35 minutes.
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Let the cake cool in the pan for 10 minutes, then turn onto a rack and cool completely. Transfer the cake to a work surface. Trim off the top using a serrated knife.
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Using the same pan (unless you have another), repeat the steps above. We made 3 layers of cake from the box mix. As the cakes are cooling, make the peanut butter frosting.
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In a medium bowl, combine the butter and peanut butter, beating with a mixer until smooth. Alternate adding the confectioners’ sugar and milk, beating after each addition, until the frosting is fluffy and smooth.
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Carefully place a cake layer on a plate or stand.
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Spread a thick layer of peanut butter frosting on the bottom cake layer, then spread the jam or jelly on top of the frosting. Top with a second cake layer. You can stop here if you’d like a peanut butter and jelly sandwich cake with only 2 layers, or continue like we did, and frost the second layer with peanut butter frosting and jelly, then top with the remaining cake layer.
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Cut the cake in half diagonally and then each half into fourths to yield 8 small slices of cake. Watch those childhood dreams come true as you enjoy this tasty treat!