1. Preheat the oven to 375 degrees.
2. Mix the sugar and butter just until combined.
3. Add the egg and milk and mix until combined.
4. Combine the dry ingredients in a separate bowl, then add to the wet ingredients. Mix until well combined.
5. Drop level tablespoons of cookie dough onto a lined cookie sheet.
6. Bake for 6-8 minutes, then remove to cool. Allow the cookies to sit on the cookie sheet for 2-3 minutes, then remove to a cooling rack.
7. To make the frosting, beat the butter and shortening until smooth.
8. Add about half of the powdered sugar and the cocoa powder and mix until smooth.
9. Add the vanilla extract and 2 tablespoons of water and mix until smooth.
10. Add the remaining powdered sugar and mix until smooth.
11. Add the additional tablespoon of water, if needed, then mix until smooth.
12. Create cookie sandwiches with two cookies at a time. Pipe frosting onto one cookie, then add the second. Add sprinkles around the sides of the cookie so that they can stick to the frosting.
13. Store cookies in an air tight container at room temperature. Best for 2-3 days.