Preheat the oven to 350 degrees. Grease a standard 12-cup muffin tin (or line with paper liners) and set aside.

Whisk the flour and baking powder in a medium bowl until combined. Whisk the egg in a separate medium bowl until well-combined and light-colored, about 20 seconds. Add the sugar and whisk vigorously until thick, about 30 seconds. Add the melted butter in 2 or 3 additions, whisking to combine after each addition. Add the sour cream in 2 additions, whisking just to combine.

Add the berries to the dry ingredients and gently toss just to combine. Add the sour cream mixture and fold with a rubber spatula until the batter comes together and the berries are evenly distributed, 25 to 30 seconds. Small spots of flour may remain and the batter will be very thick. Do not over mix.

Using a large spoon or a cookie scoop sprayed with nonstick cooking spray, divide the batter amount the muffin cups.Fill them full if you want a pretty muffin that falls over the edges.

Combine streusel mixture in a bowl. Sprinkle on top of the muffins.

Bake until the muffins are light golden brown and a toothpick or thin knife inserted into the center of a muffin comes out clean, 25 to 30 minutes, rotating the pan from front to back halfway through the baking time. Immediately remove muffins to a wire rack and cool for at least 5 minutes. Serve immediately or at room temperature. Store leftovers in an airtight container at room temperature.