Lightly spritz the liners and tin with cooking spray and set aside.
Place softened butter and granulated sugar in the bowl of a mixer fitted with the whisk attachment.
Blend until combined.
Add eggs, peanut butter, and vanilla to the bowl and continue to whisk until combined.
In a separate bowl, add flour, baking powder, baking soda, and salt. Whisk to combine.
Add the dry ingredients to the bowl with the peanut butter mixture, swap out the whisk attachment for the paddle, and blend until combined.
Add the milk and chopped nuts to the mixture and continue to combine until the batter is blended and smooth.
Divide the batter between the liners in the cupcake tin, filling each ⅔ of the way full.
Place the tin in the oven and bake for 25-30 minutes or until cooked through. Remove from the oven and set aside on a wire rack for 30 minutes, or until completely cooled.
Place the marshmallow fluff in a piping bag fitted with a large round tip.
Once the cupcakes have cooled, pipe the fluff into the center of each. Don't worry about covering the cupcakes completely as the fluff will spread. Once all the cupcakes have fluff topping, place them into the fridge to rest and chill for 30 minutes.
Once rested, use a brûlée torch to toast the fluff on each cupcake.