Preheat oven to 350°F. Grease a standard-sized 12-cup muffin tin and set aside.
Whisk together flour, baking powder, and salt in a medium bowl.
In a separate medium bowl, whisk egg until evenly combined and pale yellow in color. Add the sugar and lemon zest and whisk vigorously until thick and well combined. Add half of the butter, whisk well to combine, and then whisk in the remaining butter. Add sour cream in two additions, whisking to combine after each addition.
Add berries to the dry ingredients and toss gently to combine. Add the wet ingredients to the dry and fold with a spatula until the batter comes together – it’s okay if some bits of the flour are not totally mixed into the thick batter. Do not overmix.
Spoon the batter between the prepared muffin cups. Bake until the muffins are just set and a skewer inserted in the center of a muffin comes out clean, about 25 to 30 minutes.
Let muffins sit in the pan for five minutes before turning them out on a wire rack. Serve warm or at room temperature.