Peanut Butter Layer Cake:
Position a rack in the center of the oven and preheat the oven to 350 degrees F. Generously spray three 8-inch round cake pans with cooking spray and line the bottom of each with a parchment paper circle. Spray the parchment too.
In a large bowl, whisk together the flour, brown sugar, baking powder, baking soda, and salt. Add peanut butter, buttermilk, water, canola oil, eggs, and vanilla. Use a rubber spatula to mix the wet ingredients into the dry ingredients until just combined.
Divide evenly between the three cake pans and bake for 30 to 35 minutes, or until the top of each cake bounces back when gently pressed. A skewer inserted into the center of the cake should come out with a few crumbs attached. Cool completely in the pans on a wire rack before frosting.
Strawberry Jelly Swiss Meringue Frosting:
Whisk together egg whites and sugar in the top pan of a double boiler (or, make your own by setting a heatproof bowl on top of a saucepan filled with an inch or two of simmering water—make sure the bottom of the bowl does not touch the water). Cook over medium-high heat, whisking intermittently, until the egg mixture registers 160 degrees F on a candy thermometer.
Once hot, immediately pour into the bowl of a stand mixer fitted with the whisk attachment. Beat on high speed for 8 to 10 minutes, until the mixture is light, fluffy, and holds medium-stiff peaks. When done, the outside of the mixer bowl should be at room temperature. Scrape down the bottom and sides of the bowl, and the whisk attachment. Swap the whisk attachment for the paddle attachment.
With the mixer on low, add the butter, a few tablespoons at a time. Once incorporated, increase to medium-high and beat until the buttercream is silky smooth, 3 to 5 minutes. Scrape down the bottom and sides of the bowl. With the mixer on low, gradually add the jelly, 1 to 2 tablespoons at a time. Once incorporated, increase to medium-high and beat until silky smooth and pale pink-purple, 2 to 3 minutes more.
For this recipe, you can use any brand of peanut butter you want—just be sure it doesn’t have any added sugar in it because the cake itself is already going to be pretty sweet. But you can definitely use either creamy or crunchy peanut butter. Creamy peanut butter will result in a soft and tender cake crumb, while crunchy peanut butter will result in a crumb more similar to a carrot cake or banana bread recipe with nuts in it.
I mentioned before that you can use any kind of jam or jelly you want, too. The frosting is infinitely customizable; simply make the Swiss meringue buttercream, then mix in your favorite jam or jelly after to flavor it. I especially love the combo of strawberry and blackberry jam with the peanut butter.
When making the frosting, make sure the butter is completely at room temperature (ideally at 70 degrees). Swiss meringue buttercream has a tendency to curdle or separate if the butter is too cold. If you find that the buttercream has curdled once all the butter has been added, scoop out ¼ cup (no need to be precise—you can just eyeball it!), microwave it until melted but not hot, then pour it into the buttercream and beat on medium for 2 to 3 minutes. The buttercream should turn silky and creamy.