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Preheat oven to 350 degrees and grease a 9x13 baking dish.
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Sprinkle peanuts and 1/2 cup of peanut butter chips all over the bottom of the baking dish. Set aside.
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In a large bowl with a stand mixer (or a hand mixer), cream the peanut butter and vegetable oil together until well combined.
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Add in the granulated sugar and brown sugar, creaming the mixture with your mixer until light and fluffy, about 1-2 minutes (this helps your cake be light and not nearly as dense).
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Add in the eggs, one at a time, beating well after each addition.
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In a separate bowl, sift together the flour, cornstarch, baking soda, baking powder and salt.
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Alternately, beginning and ending with the flour mixture, mix the flour mixture and the buttermilk in with the peanut butter batter, until just combined.
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Add in the vanilla extract and mix until combined.
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Pour the batter into the baking dish, smoothing in an even layer. Set aside.
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In a separate bowl, whisk (by hand or with a mixecream cheese, melted butter, powdered sugar and half & half together.
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Once combined, drop the batter by spoonfuls all over the top of the peanut butter cake batter.
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With a knife, swirl the cream cheese filling and the peanut butter cake batter together.
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Sprinkle the remaining peanut butter chips on top.
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Bake for 35-37 minutes. The cake should be set on top, but the center should jiggle slightly when the cake pan is gently moved.
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Allow the peanut butter earthquake cake to cool before serving. I prefer this cake cold, but you can serve warm, at room temperature or cold.
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Enjoy! Store leftovers in the fridge.