How to Make It
Preheat oven to 350°F. Line a 12-cup standard muffin tin with paper liners (or coat with nonstick spray and dust with flour).
Whisk baking powder, salt, and 2 cups flour together in a medium bowl. Using an electric mixer, beat butter and both sugars together in a large bowl on medium-high until light and fluffy, about 5 minutes. Add egg and beat to combine, scraping down sides of bowl; beat in vanilla.
With mixer on low, add dry ingredients in three additions, alternating with milk, and beginning and ending with dry ingredients, beating to combine after each addition. Scrape down sides of bowl as necessary and beat to combine.
Toss berries with remaining 2 teaspoons flour until coated; fold berries into batter. Divide batter among prepared muffin cups and bake, rotating tin halfway through, until a toothpick inserted in the center comes out clean, 25–30 minutes. Let cool slightly before serving.