Preheat the oven to 350 degrees F (175 degrees C). Line 2 muffin tins with paper liners.

Combine brown sugar, butter, and peanut butter in a medium bowl. Beat with an electric mixer until creamy. Beat in eggs one at a time until combined.

Combine flour and baking powder in a separate bowl. Slowly beat into the creamed mixture. Pour in milk and water; stir until batter is thick and smooth. Spoon batter into the prepared muffin tins. Press 2 mini peanut butter cups into each cupcake.

Bake in the preheated oven until edges are firm and begin to pull away from the sides of the tin, 20 to 25 minutes.

Pile 15 to 20 chocolate chips over each cupcake. Return to the oven until chocolate chips start to melt, about 1 minute. Spread melted chocolate over the cupcakes and sprinkle with chopped peanuts.