Preheat oven to 350 F. Line a muffin pan with liners.

In a large bowl, sift the flour, cocoa, baking powder, baking soda, and salt together. Set aside.

In a medium sized bowl, whisk the egg and sugar with the oil until smooth. Add the vanilla then pour half of the wet mixture along with the buttermilk into the dry ingredients. Whisk until just combined then add the remainder of the wet mixture. Stir until just combined, being careful not to over mix. 

Pour the batter into the cupcake tin. Fill each cup about 2/3 full. 

Bake 17-21 minutes or until a toothpick comes out clean. Allow to cool 10 minutes in the cupcake tin then transfer to a wire rack to cool. Cool completely before frosting. 

Beat the peanut butter and room temperature butter on medium. Scrape down the sides and add the vanilla and salt.

With the mixer on low speed, add the powdered sugar and beat for another minute. Add the heavy cream and beat until buttercream is combined and spreadable. Use as soon as possible.