Preheat oven to 350F and grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk flour, baking powder, and salt. Set aside.
Using a stand mixer fitted with the paddle attachment, beat butter & peanut butter until completely smooth. Add sugar and beat on med-high (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of buttermilk). Fully incorporating after each addition. Do not overmix.
Spread batter evenly into prepared pans. Smooth the tops with a spatula.
Bake for approx. 25-30mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack.
Peanut Butter Frosting:
Beat butter until creamy. Add peanut butter and beat until well combined (2-3 mins).
Add sifted powdered sugar in one 1/2 cup at a time and beat until combined. Add vanilla.
Add cream one Tbsp at a time until desired consistency. Beat on high for 3 minutes until fluffy.
Place one layer of cake on a cake stand or serving plate. Top with 1/2 cup of frosting and spread evenly. Top with 3 Tbsp of jam. Repeat with remaining layers, then stack each on top of the other.**
Top with chopped peanuts if desired.
* I used seedless jam as I like the smooth texture, but you could use one with seeds if you prefer. ** The jam makes stacking the layers difficult. I recommend frosting each layer of cake separately and then stacking and also placing 3 cake skewers into the cake once stacked for additional support.