Prep the pan: Line a 12-cup muffin tin with paper liners or spray with nonstick baking spray that contains flour.
Mix dry ingredients: In a medium bowl, whisk together 2 ½ cups flour, baking powder, and kosher salt.
2 ½ cups all-purpose flour, 2 ½ teaspoons baking powder, 1 teaspoon kosher salt
Mix wet ingredients: In a large bowl, whisk together the sugar and eggs until thick and pale, about 1 minute. Slowly whisk in the melted butter until combined. Add buttermilk and vanilla extract and stir until smooth.
1 cup granulated sugar, 2 large eggs, ½ cup unsalted butter, 1 cup buttermilk, 2 teaspoons vanilla extract
Prepare berries: Toss the strawberries, blueberries, and raspberries with 2 teaspoons of flour to prevent them from sinking.
½ cup diced strawberries, ½ cup blueberries, ½ cup raspberries
Combine: Gently fold the dry ingredients and floured berries into the wet mixture until just combined. Do not overmix—some lumps and streaks of flour are okay.
Rest: Cover the bowl with plastic wrap and let the batter rest for 15 minutes while you preheat the oven to 425°F (218°C).
Fill and bake: Divide the batter evenly among the muffin cups using a scoop. Bake for 15–20 minutes, or until golden brown and a toothpick inserted into the center comes out clean.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.