Line a 12-serving cupcake pan with 10 paper liners. Set aside.
Add the flour and baking powder to a small mixing bowl and whisk to combine. Set aside.
Add the butter, sugar, egg, and peppermint extract to a large mixing bowl and beat with an electric mixer until smooth and creamy.
Add the dry ingredients and milk to the mixing bowl and mix until smooth. Add 2 Tbsp. milk first, and then the remainder if needed to get to the right consistency.
Add some green food coloring to the mixing bowl and mix until fully incorporated. Scrape down the sides of the mixing bowl with a rubber spatula and mix once more to ensure all the ingredients are fully incorporated. Add more food coloring if necessary until the desired shade of green is reached - ideally a light, minty shade of green.
Pour the batter evenly into the prepared cupcake pan until each liner is about ¾ full.
Bake the cupcakes in the preheated oven for 24 minutes, or until a toothpick inserted into the center of one of the cupcakes comes out clean.
Transfer the cupcakes onto a wire cooling rack to cool completely.
Directions for Frosting:
Add the butter, vanilla, powdered sugar, and milk to a large mixing bowl and mix on low speed until fully incorporated. Increase the mixer speed and beat the frosting until smooth and fluffy.
Scoop the frosting into a large decorating bag fitted with a large star tip.
Pipe a generous swirl of frosting onto each cupcake.
Top each cupcake with sprinkles and a straw piece so that each cupcake mimics a miniature shamrock shake.
Store any leftover cupcakes in an airtight container for up to three days.
Notes
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