Make the cupcakes: Position a rack in the middle of the oven and preheat to 350 degrees F using the convection setting, if available. Line two 12-cup muffin pans with paper liners. Put the butter, cocoa powder and 3/4 cup water in a microwave-safe bowl, cover with plastic wrap and microwave until the butter melts, about 2 minutes. Whisk to combine, then whisk in the brown sugar.
Whisk the flour, baking powder, baking soda and salt in a large bowl. Whisk in the warm cocoa mixture. In another bowl, beat the buttermilk, eggs and vanilla; stir into the batter until just combined but don't overmix.
Divide the batter among the prepared cups, filling each three-quarters of the way. Bake until the cupcakes are slightly domed and spring back when lightly pressed, 25 to 30 minutes. Let cool in the pans 10 minutes, then transfer to racks to cool completely.
Make the brittle: Line a baking sheet with a silicone mat or parchment paper. Place the granulated sugar in a small saucepan over medium-high heat, bring to a boil and cook, swirling the pan but not stirring, until it turns amber. Remove from the heat; stir in the peanut butter and butterscotch chips until smooth. Immediately pour the mixture onto the prepared baking sheet, smooth into a thin layer with a spatula and let cool. Break the brittle into pieces, place in a resealable plastic bag and crush with a rolling pin.
Make the frosting: Bring the cream to a simmer in a small saucepan. Remove from the heat, add the peanut butter chips and let stand about 5 minutes. Whisk until smooth. Place the pan over a bowl of ice, stirring occasionally, until the frosting is spreadable, about 10 minutes. Spread on the cupcakes or transfer to a pastry bag with a large round tip and pipe on top; sprinkle with the brittle.