Preheat oven to 350°F. Line a 12-cup muffin pan with paper liners.
Whisk flour, sugar, cocoa, baking powder, baking soda, and salt in a large bowl.
Whisk buttermilk, oil, eggs, and vanilla in a separate bowl until smooth.
Pour the wet mixture into the dry ingredients. Stir just until combined.
Stream in hot coffee and mix until the batter is silky and pourable.
Divide batter among liners, about two-thirds full. Sprinkle in chopped candy and gently push under the surface.
Bake 17–19 minutes, until a tester comes out clean. Cool in the pan 5 minutes, then fully on a rack.
Beat butter and peanut butter on medium-high until fluffy, about 2 minutes.
Add powdered sugar, vanilla, and salt. Beat on low, then add milk to reach a creamy piping consistency.
Pipe or spread frosting on cooled cupcakes. Drizzle with caramel and finish with peanuts and more chopped candy.