Preheat the oven to 350 degrees. Place parchment on two cookie sheets or spray with non-stick spray. Set aside.
In the bowl of a stand mixer, combine the softened butter and sugars. Cream together until smooth.
Add the eggs, vanilla, baking soda, and salt into the stand mixer bowl. Mix well - scraping down the sides and mixing again.
Add in the flour and cocoa powder and mix well. The mixture will be thick.
Scoop out a golf-ball sized amount of dough and flatten slightly into a disc. Place on prepared cookie sheets, leaving space between the cookies for spreading during baking. Top with about a teaspoon's worth of sprinkles.
Bake in preheated oven for 10-12 minutes or until edges are set.
While the cookies are baking, make the buttercream frosting.
In the bowl of a stand mixer, combine the softened butter, salt, vanilla, and about half of the powdered sugar. Cream until smooth.
Add in the remaining powdered sugar and about 1 tablespoon of milk. While it mixes, if you need to add more milk to thin, only add 1 tablespoon at a time until thoroughly mixed.
Add in the neon green food coloring and whip on high speed for about a minute. Scrape the sides and mix again.
Transfer about a cup of frosting to a microwave safe dish and heat for about 30 seconds, or until the frosting melts just slightly, and stir.
Fill the syringe with the melted frosting.
Once cookies are removed from the oven, immediately inject the melted buttercream into the center of the cookie while it’s still hot and pliable. If you wait too long, the cookie will harden, preventing the frosting from filling the center of the cookie. The frosting should ooze out the cracks and overflow out of the cookie.
Refill the syringe as needed and fill all the cookies. Follow up with a few more sprinkles, if desired.
Allow the cookies to cool and for the frosting to set back up. Enjoy!
