Prepare the Chocolate Cupcakes: Preheat oven to 350°F. Line 2 (12-cup) muffin pans with paper baking cups. Whisk together flour, cocoa, baking powder, baking soda, and salt in a large bowl until thoroughly combined. Beat granulated sugar, brown sugar, and butter in bowl of a stand mixer fitted with a paddle attachment on medium speed until well combined, 2 to 3 minutes. Add eggs, 1 at a time, beating until just incorporated after each addition. Add vanilla; beat until just incorporated, about 30 seconds. Add flour mixture to butter mixture in 3 additions, alternating with buttermilk, beginning and ending with flour mixture, beating on low speed until blended after each addition. Increase speed to medium, and beat batter until smooth and well combined, 1 to 2 minutes, scraping down sides with a rubber spatula as needed. Spoon 1/4 cup batter into each baking cup.

Bake in preheated oven until a wooden toothpick inserted into center comes out clean, 20 to 22 minutes. Cool cupcakes in pans 5 minutes. Remove from pans to wire racks, and let cool completely, about 30 minutes.

Prepare the Peanut Butter Buttercream: Beat powdered sugar and butter in bowl of a stand mixer fitted with a paddle attachment on low speed until well blended, about 1 minute. Increase speed to medium, and beat until fluffy, 2 to 3 minutes. Scrape down sides of bowl. Reduce speed to low, and beat in peanut butter, vanilla, and salt until just incorporated, about 30 seconds. Increase speed to medium, and beat until light and fluffy, about 2 minutes. Reduce speed to low, and gradually beat in heavy cream until just incorporated, about 30 seconds. Increase speed to medium, and beat until light and fluffy, about 2 minutes. Transfer buttercream to a piping bag fitted with a star attachment.

Pipe buttercream (about 1/3 cup) in a spiral on top of each cupcake. Sprinkle each with 1/2 tablespoon chopped peanuts. Serve immediately or store in an airtight container at room temperature up to 2 days.