Preheat your oven to 350 degrees Fahrenheit.
In a medium bowl, sift together 1 1/2 cups of all-purpose flour, 2 teaspoons of baking powder, 1/2 teaspoon of baking soda, and 1/2 teaspoon of salt.
In a large mixing bowl, combine 1/3 cup of natural unsweetened peanut butter, 1/3 cup of coconut oil, and 1 cup of brown sugar. Beat until creamy.
Add 2 large eggs and 1 teaspoon of vanilla extract. Beat until just combined.
Alternately add the dry mixture and 1 cup of buttermilk to the wet mixture, beating until combined.
Grease a 9X3 inch round baking pan and pour the batter into it.
Bake for about 30 minutes, checking with a toothpick for doneness.
Let the cake cool for 5 minutes before inverting it onto a wire rack to cool completely.
For the frosting, combine 1 cup of natural unsweetened peanut butter and 1/2 cup of softened unsalted butter. Beat until smooth.
Add 3 cups of powdered sugar gradually, beating until combined, then add heavy cream to reach spreadable consistency.
Cut the cooled cake in half and frost the bottom layer, then stack and frost the top layer.