Preheat oven to 375 F.

Line cupcake pan.

Combine cupcake ingredients in mixing bowl and beat until well combined, making sure to scrape down the sides as needed.

Add about 2 tbsp of batter to liners so they're about 3/4 full.

Reduce oven to 350 F.

Bake cupcakes for about 18-20 minutes until toothpick comes clean.

Let set in pan for a couple of minutes before transferring to oven rack to finish cooling.

Core cupcakes and pipe in fluff until each is full.

Combine heavy cream, confectioners' sugar, and pudding mix in mixing bowl and beat until stiff peaks form.

Pipe frosting onto cupcakes and sprinkle with sprinkles, chopped walnuts, and mini marshmallows.