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 a 24-cup mini muffin pan with paper liners. Combine the butter, cocoa powder and 1/4 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, whisk the buttermilk, egg 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 spring back when touched, about 20 minutes. Let cool in the pan 5 minutes, then transfer to a rack to cool completely.
Make the topping: Bring the cream to a simmer in a small saucepan, then pour over the peanut butter chips in a bowl and let stand until the chips melt, about 5 minutes. Whisk until smooth. Refrigerate until the topping is set, about 10 minutes, then beat with a mixer until fluffy. Transfer to a pastry bag with a 1-inch round tip and pipe peaks onto each cupcake. Place in the freezer while you make the glaze.
Make the glaze: Put the butter, chocolate and 3 tablespoons hot water in a small, deep microwave-safe bowl, cover with plastic wrap and microwave on 50 percent power until the chocolate melts, about 2 minutes. Whisk until smooth. Dip the frozen peaks of each cupcake into the glaze, letting the excess drip off. Refrigerate until set, about 5 minutes.