Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease well.
Make the streusel: In a small bowl, combine flour, brown sugar, and cinnamon. Cut in the cold butter with a fork or pastry cutter until the mixture forms pea-sized crumbs. Refrigerate while you prepare the batter.
Whisk dry ingredients: In a large bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, baking soda, and salt.
Whisk wet ingredients: In a separate bowl, whisk the buttermilk, melted (cooled) butter, eggs, and vanilla until smooth.
Combine: Pour the wet ingredients into the dry and fold gently with a spatula until just combined; a few small flour streaks are fine. Do not overmix.
Fold in strawberries: Toss chopped strawberries with a teaspoon of flour if very juicy, then gently fold into the batter just until evenly distributed.
Fill and top: Divide batter among muffin cups, about 3/4 full. Sprinkle the chilled streusel evenly over each muffin, pressing very lightly so it adheres.
Bake: Bake 18–22 minutes, until the tops are golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool: Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Optional glaze: Whisk powdered sugar, milk, and vanilla to a drizzleable consistency. Zigzag over cooled muffins and serve.