For the Cake

• Set oven rack to the middle position and preheat oven to 350°F. Spray an 8x8-inch baking pan with nonstick spray. Set aside.
• In the bowl of a stand mixer (or using a hand mixer), slowly beat together the cake mix, oil, peanut butter, eggs, vanilla, and buttermilk. Mix for 1 minute on low until ingredients are combined. Turn mixer to medium speed and mix for 1-2 more minutes, until smooth and the dry ingredients have been incorporated into the batter. Pour the batter into the prepared baking pan; set aside.
  15.25 ounces white cake mix, ¼ cup vegetable oil, ½ cup smooth peanut butter, 3 large eggs, 1 teaspoon pure vanilla extract, 1 cup buttermilk
• In a small bowl, mix together the peanut butter, sugar, oil, and vanilla until smooth; set aside.
  ⅓ cup smooth peanut butter, ¼ cup powdered sugar, 1-2 tablespoons vegetable oil, ½ teaspoon pure vanilla extract
• In another small bowl, whisk together the jelly (jam) and the hot water until loosened and almost smooth.
  ⅓ cup grape jelly or jam, ¾ teaspoon hot water
• Place alternating dollops of the grape jelly and the peanut butter mixtures, about ½-1 inch apart, over the entire cake. Drag the tip of a knife through the dollops to create a swirl pattern throughout the entire cake and from edge-to-edge.
• Bake at 350°F for 45-60 minutes, until a toothpick or cake tester inserted in the middle comes out almost clean. Transfer cake to a cooling rack to cool completely before cutting.