Preheat oven to 350°F (175°C). Grease 12 mini muffin tin cups.
In a large bowl, sift together flour, baking powder, sugar, and salt.
In a separate bowl, mix softened butter until creamy. Add eggs, one at a time, blending fully after each addition.
Pour in milk and vanilla extract, stirring to combine.
Gradually add the dry ingredients to the wet mixture. Mix until fully incorporated but do not overbeat.
Divide the batter evenly among the prepared muffin cups, filling each about 1/3 to 1/2 full.
Bake for 15-20 minutes, or until a toothpick inserted in the center comes out clean.
Allow cupcakes to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.
Once cooled, pipe or spread a generous amount of frosting on each cupcake.
Top each with a handful of rainbow sprinkles for decoration. Chill in the refrigerator for about 15 minutes to set the frosting before serving.