Preheat the oven to 360 degrees F. Prepare three 8-inch or four 6-inch round cake pans (butter or spray, line bottom with parchment paper, butter or spray paper and dust with flour).

In a bowl of electric mixer, combine all the dry ingredients.

Combine eggs, buttermilk, water, oil and vanilla in a measuring cup and lightly beat with a fork.

Add wet ingredients to the dry ingredients, and mix on medium speed for two minutes. Batter will be thin. Scrape sides and mix for another 30 seconds. Divide batter evenly among prepared pans (I like to use a kitchen scale to ensure the batter is evenly distributed).

Bake for 18 minutes (you may need more time depending on your oven), until toothpick comes out almost clean. Cool on wire racks for 10-15 minutes, then gently invert onto racks until cooled completely.

When cakes are completely cooled, I like to wrap each cake layer individually with plastic wrap and stick it in the refrigerator or freezer until I’m ready to frost. The cake will be easier to work with if it’s cooled a bit. If I’m making the cake a day or two before I actually need to frost it, I put it in the freezer to ensure freshness. If I’m frosting same-day, I’ll just put it in the refrigerator to chill until needed.

In the bowl of electric mixer fitted with the paddle attachment, combine the butter and peanut butter. Beat on medium-high speed until smooth, about 45 seconds.

Add the powdered sugar to the bowl and mix in, slowly at first until incorporated, then increasing the mixer speed to high. Blend in the vanilla extract, salt and heavy whipping cream, and continue to whip on high speed until very fluffy, about 4-5 minutes, scraping down the sides of the bowl as needed. 

Use a wooden spoon to mix frosting to get rid of air bubbles.