For the Food Colors and Garnish
For the Cookies
Preheat oven to 350˚F.
Line two baking sheets with parchment paper and set aside.
Whisk eggs in a large bowl; add the sugar and whisk to combine.
Whisk in milk, butter, and vanilla until smooth.
In a separate bowl, whisk together the flour, baking powder, and salt.
With an electric mixer on low, add flour mixture to the egg mixture one tablespoon at a time; continue to mix until thoroughly incorporated.
Remove the mixing bowl from the mixer and put it in the fridge for 15 minutes.
Remove the mixing bowl from the fridge.
Lightly grease your hands with cooking spray, pinch off a little dough, and form into small balls about 1 inch in size.
Place the cookie balls on the baking sheets and bake for 12 to 15 minutes or until lightly browned on the bottom.
Remove from oven and let them rest on a cooling rack for 8 to 10 minutes or until cool enough to handle.
Using a sharp paring knife, carefully cut out the bottoms of each cookie and make a well in the center of the cookie to fit the filling.
Reserve the cookie crumbs in a mixing bowl.
For the Filling
In the bowl with the cookie crumbs, add the Nutella and milk; mix until thoroughly combined. If needed, add more milk for a creamier mixture.
Fill the cookies with the Nutella mixture and sandwich 2 cookies together. Try to match the cookies with the same shape cookie and/or size.
For the Food Colors and Garnish
Place half of the peach liqueur in one small bowl and the rest of the liqueur in another small bowl.
Drop a few drops of red food color in one of the bowls with the liqueur and yellow food color in the other.
Dip one side of the cookie in red food coloring and the other side in yellow. Let them rest on a sheet of wax paper for 5 minutes before adding the sugar. I dip about five cookies in the colors and then go back to the first one and sprinkle with sugar. You can also roll them in the sugar.
Place the finished cookies on a sheet of wax paper to dry and continue to work with the rest of the cookies.
Once the cookies have dried, insert a couple of mint leaves in each prepared cookie and transfer to a cookie tray.