Preheat oven to 350F and grease a 15x10x1 jelly roll pan. (I used this blue one)
In a medium-sized bowl, whisk together flour, sugars, baking soda, and salt. Set aside.
2 cups flour, 1 cup sugar, 1 cup brown sugar, ¾ tsp baking soda, ½ tsp salt
In a medium-sized saucepan over medium heat, combine butter, water, and oil.
¾ cup stick butter, ¼ cup oil, 1 cup water
Stir occasionally until butter is completely melted and bring to a boil. Immediately remove from heat and add peanut butter, stir until peanut butter is dissolved and mixture is smooth.
½ cup creamy peanut butter
Gradually stir flour mixture into liquid mixture until completely combined.
Allow mixture to cool for 5-10 minutes. Meanwhile, whisk together buttermilk, eggs, and vanilla in a separate medium-sized bowl.
½ cup buttermilk, 2 large eggs, 1 ½ teaspoons vanilla extract
Gradually stir buttermilk mixture into flour/peanut butter mixture, stirring until just combined.
Pour evenly into prepared jelly roll pan and bake on 350F for 18-22 minutes.
Remove from heat and allow to cool completely in pan. Meanwhile, prepare your frosting.
Peanut Butter Frosting
Use an electric mixer to beat together butter and peanut butter until smooth. Gradually add powdered sugar until combined.
½ cup creamy peanut butter, ½ cup unsalted butter, 1 cup powdered sugar
Stir in salt and vanilla.
½ teaspoon vanilla extract, ½ teaspoon salt
Gradually, with mixer on low speed, add heavy cream.
1 ½ Tablespoon heavy cream
Increase speed to high and beat for one minute until well-combined (pause mid-way to scrape down the sides and bottom of the bowl).
Scrape peanut butter frosting into a separate bowl, and then use the now-empty bowl to prepare your chocolate frosting.
In a microwave-safe bowl, microwave chocolate chips at 15 second intervals, stirring between intervals, until chips are completely melted. Set aside and allow to cool slightly.
½ cup semisweet chocolate chips
In KitchenAid (or with electric hand mixer) cream butter.
½ cup unsalted butter, ½ tsp vanilla extract
Add sugar gradually until well-combined (scrape down bottom and sides of bowl as needed).
1 cup powdered sugar
Stir the chocolate (it should be melted but should not be too hot) and add it to your butter/sugar mixture.
Beat on medium-speed until well-combined.
Gradually, with mixer on low speed, add heavy cream.
1 ½ Tablespoon heavy cream
Increase speed to high and beat for one minute until well-combined (pause mid-way to scrape down the sides and bottom of the bowl).
Drop frosting by spoonful, alternating chocolate and peanut butter, over sheet cake.