Make the strawberry reduction: Hull and roughly chop the strawberries. Place them in a small saucepan over medium heat. Cook, stirring occasionally, until they break down and the mixture thickens into a deep, concentrated red – about 8-10 minutes. You should have about 1/3 cup left. Set aside to cool completely. Do not skip the cooling – warm puree will melt the butter in your batter.
Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
Whisk dry ingredients: In a medium bowl, whisk together the flour, baking powder, and salt.
Cream the butter and sugar: In a large bowl, beat the softened butter and sugar together until light and fluffy – about 3 minutes. Scrape the bowl down well. It should look pale and airy.
Add the eggs and vanilla: Add the eggs one at a time, beating well after each addition. Beat in the vanilla. The mixture will look smooth, glossy, and slightly thicker.
Fold in the strawberry puree: Add the cooled strawberry reduction and gently fold it in by hand with a rubber spatula until just combined. The batter will turn pale pink. Do not overmix – a few streaks are fine.
Alternate flour and milk: Add half the flour mixture, then half the milk, mixing on low speed until just combined. Repeat with remaining flour and milk. Stop the mixer the moment the batter comes together.
Fill and bake: Divide the batter evenly among the 12 liners, filling each about 2/3 full. Bake for 18-22 minutes. A toothpick inserted into the center should come out with a few moist crumbs, not wet batter. The edges should be just barely golden and beginning to pull away from the paper liner.
Cool completely: Let the cupcakes cool in the tin for 5 minutes, then transfer to a wire rack to cool completely. Wait until fully room temperature before frosting. Enjoy!