Preheat the oven to 350 degrees. Line a couple standard muffin tins with cupcake liners and set aside.
For the cupcakes - in the bowl of a stand mixer, combine the sugar, flour, cocoa, baking powder, baking soda, and salt. Whisk to combine.
Add in the vanilla, eggs, milk and oil. Mix on low until combined and scrape down the sides often.
Using a measuring cup or similar, scoop the cake batter evenly into the prepared cupcake tins.
Bake in the preheated oven for 14-16 minutes or until the center is set. Allow to cool completely before decorating.
For the frosting - in the bowl of a stand mixer, combine the softened butter and about half of the powdered sugar.
Mix on low until combined, scraping down the sides often.
Add in the remaining powdered sugar, salt and vanilla. Mix on low until combined.
Add in the milk and mix until combined, scraping down the sides often.
Add in the green food coloring - starting with just a little and adding more if needed to achieve the desired shade of green.
Transfer the frosting to a piping bag fitted with a large star tip.
To assemble - once the cupcakes are cooled, pipe a large mound of frosting over each cupcake.
Immediately decorate with the sprinkles and insert a Peeps marshmallow to set in the center.