Start by preparing your buttercream. Place all ingredients in a large bowl: softened butter, confectioners’ sugar and milk (start with 1 tbsp and add another, slowly if you feel like the buttercream is too dry).
Beat all buttercream ingredients with an electric mixer, starting on slow speed and gradually increasing to high.
Once all ingredients are smooth and form semi-stiff peaks, scoop into a piping bag and frost each cupcake.
Pour coarse sugar/shiny sprinkles into a bowl. Dip each frosted cupcake into the bowl of sprinkles.
Attach Wilton tip #3 to your bladck decorating frosting and pipe out 1-2 curvy lines on each cupcake.
Add 4-8 M&Ms sideways onto the black, curvy lines on each cupcake (depending on how many lines you have on each one). You’ll want about 4-5 M&Ms for each “string of lights.”