In a large bowl or stand mixer, beat butter until soft and fluffy, 2-3 minutes. Make sure you scrape the sides and bottom of the bowl.

Add sugar and brown sugar and beat for another minute.

Add the egg, sour cream, and 1 teaspoon vanilla. Beat well.

Add the flour and cocoa powder but do not stir.

Use a small spoon to stir the baking powder, baking soda, and salt into the flour and cocoa powder.

Beat until just barely combined, scraping sides and bottom as necessary.

Use a cookie scoop to form balls of dough. You want them to be about 2 inches across.

Place the shaped cookies on a baking sheet or large plate. Cover with plastic wrap and chill for 1-3 hours. (The more thoroughly you chill, the puffier your cookies will be.)

Preheat your oven to 350 degrees F. Line a few baking sheets with parchment paper or a silpat.

Place the chilled cookies on a baking sheet with about 2 inches between each cookie.

Bake in the preheated oven for about 10-12 minutes, until the top has just barely lost its shine.

Remove from the oven and let cool 5 minutes on the pan. Transfer to a cooling rack and let cool completely. Repeat until all the cookies have been made.

Meanwhile, prepare the frosting. In a large bowl or stand mixer, beat the butter until smooth. Add the peanut butter and beat until smooth, scraping sides and bottom.

Add the powdered sugar, milk, and 2 teaspoons vanilla. Beat until combined, scraping sides and bottom. 

Frost the cookies once the have cooled completely. 

Immediately after frosting, sprinkle each cookie with chopped pretzels and peanuts. This step is optional, but I love the crunch of flavor it adds!