1-Preheat oven to 180°C (350°F). For fan or convection ovens, reduce the temperature to 160°C (320°F). Grease and line two 8-inch cake pans to prevent sticking.

2-In a large bowl, cream together the unsalted butter, vegetable oil, white granulated sugar, light soft brown sugar, and smooth unsweetened creamy peanut butter on medium speed for about 3 minutes until the mixture is light and creamy.

3-Add the eggs one at a time on low speed, mixing well after each addition. Then, incorporate the vanilla extract and buttermilk, blending on medium speed until fully combined.

4-Gently fold the sifted dry ingredients such as all-purpose flour, cornstarch, baking soda, baking powder, and salt into the wet mixture using a spatula. Avoid overmixing to keep the batter airy.

5-Divide the batter evenly between the prepared pans and tap them lightly to remove air bubbles. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

6-Allow the cakes to cool in the pans for 15-20 minutes, then transfer them to wire racks to cool completely before adding the frosting.

7-For the frosting, combine all frosting ingredients in a large bowl or stand mixer. Mix on low speed until blended, then beat on medium-high for 10 minutes, scraping the sides halfway through. If the frosting is too firm, warm it briefly and re-whip for smoothness.

8-To assemble, level the cake tops if needed, place one layer on a stand, spread frosting on top, add the second layer, and frost the top and sides evenly. Smooth with a scraper and spatula, then decorate as desired.