Preheat oven to 350 degrees F. Line a cupcake pan with paper liners.
In a large mixing bowl combine flour, sugar, baking powder and salt. Slice the butter into pieces and add to the flour mixture. Mix on low until the mixture resembles coarse crumbs.
With the mixer on, add the eggs one at a time, scraping down the bowl after each addition. Add the milk and vanilla and mix until batter is smooth. Fold in the sprinkles with a rubber spatula.
Fill the cupcake liners about two-thirds full. Bake for 15-20 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool before frosting.
For the Frosting:
In a mixing bowl cream the butter for several minutes until light and fluffy, scraping the bowl down as needed. Gradually add the powdered sugar, milk and vanilla and mix until light and fluffy, about 2 minutes.
Frost the cupcakes (I used a Wilton 1M tip) and add sprinkles as desired.
Make Ahead Instructions: The cupcakes and the frosting can both be made ahead of time. Allow the cupcakes to cool completely, then place in an airtight container and refrigerate for up to 1 week. Refrigerate frosting separately for up to 2 weeks. Allow frosting to come to room temperature before using.Freezing Instructions: Allow cupcakes to cool completely, then store in a freezer-safe container for up to 3 months. Frost the cupcakes frozen, if desired (allowing 30 minutes for them to thaw, before serving). To freeze frosted cupcakes, I suggest flash-freezing them on a tray for 30 minutes (to help the frosting keep its shape) before placing them in a freezer safe container.