Cream butter and sugar together in a large bowl using an electric mixer (or use your stand mixer).
Add the eggs and vanilla, and beat until combined.
Then add flour, baking powder, and salt. Beat until the dough is uniform.
Carefully divide the dough into three equal parts, and place each in a separate bowl.
Add one color of gel food coloring to each bowl, a few drops at a time, until you reach your desired shades. Keep in mind that the colors will darken slightly once you bake the cookies.
Create 1 ½ - inch balls of dough and place them on your prepared baking sheet, leaving at least two inches between each cookie.
Use a flat-bottomed glass or measuring cup to gently press the balls until they are between ¼ - ½ inch thick. Spray the glass with cooking spray to avoid any stickiness.
Place the tray into the oven and bake for 8-10 minutes, or until the edges of the whoopie pies are lightly browned and the tops are no longer wet-looking.
Remove from the oven and cool completely on wire racks.
For the Frosting:
In a medium mixing bowl, cream together softened butter and powdered sugar with an electric mixer.
Mix in vanilla extract and milk. Beat with the mixer until the buttercream is smooth and creamy.
To Make Whoopie Pies:
Spread a generous amount of frosting on the back of one whoopie pie, and top with a second one to make a sandwich.
Roll the edges of the sandwich into sprinkles that you’ve placed in a shallow bowl.
Continue with the rest of the whoopie pies.
Notes
Make these any color you like! Pink, red and purple are fun for valentines day, but you can make vanilla whoopie pies in Christmas colors, Halloween colors, or your favorite colors for birthday parties or other special occasions.
Gel Food Coloring is best. When baking, you want to avoid adding unnecessary moisture to your doughs and icings. Gel food coloring is low moisture and highly pigmented, so you only need a few drops to get vibrant colors for your whoopie pies.
Use room-temperature eggs - they blend into the other ingredients more smoothly. I like to set out the butter and eggs at the same time so that they can both be at room temperature and ready to go.
Try to make the dough balls the exact same size so that your cookie sandwiches match up perfectly. A cookie scoop will help you portion the dough properly. You might also consider weighing the dough with a kitchen scale.
Don't add icing to warm cookies! Make sure that you allow the cookies to cool down completely before filling them with buttercream. Otherwise, the frosting will melt.
Make them one at a time. While it might seem easier to add the frosting to all of the whoopie pies first, then roll them in sprinkles, it's actually not. Rolling the whoopie pies into sprinkles while the frosting is freshly added will ensure that the sprinkles stick to it.