Preheat the oven to 350 degrees F. Line two muffin tin with liners. Set aside.

In a medium bowl, whisk the flour, cocoa powder, baking powder, baking soda, and kosher salt together. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugars together on medium speed for 3-4 minutes. Add the eggs, one at a time, beating for a minute or so after adding each egg. Add the vanilla extract.

Lower the speed of the mixer to low, add half of the flour mixture, and beat until it is well-incorporated. Slowly stream in the buttermilk until it is all combined. Add the remaining flour mixture and beat until it is just combined. Finish stirring everything with a spatula, if necessary.

Fill the liners ⅔ of the way full. Bake for 20-25 minutes, or until a knife/toothpick, when inserted into the center of the cupcake, comes out clean.

Allow the cupcakes to cool for 10 minutes in the pan, then remove and place them on a wire rack to finish cooling completely.

Using a stand mixer fitted with the paddle attachment, beat the butter on medium speed until it is softened (about 1 minute).

Add the peanut butter, salt, and vanilla extract and beat well.

Add 2 cups of the powdered sugar and beat on low until well combined.

Add another 2 cups of sugar along with 3 tbs of heavy cream/milk. Beat until combined.

Add the remaining 2-3 cups of sugar (depending on your desired consistency) and remaining cream/milk and beat until the frosting reaches the desired thickness.

Beat on medium speed until the frosting is smooth and fluffy (about 4 minutes).

Once the cupcakes have cooled completely, frost them.