Preheat oven to 375°. Line a 12-count muffin tin with paper liners. Set aside.
In a large bowl, using a hand mixer, cream the butter and sugar together until light and fluffy, about 2 minutes. Add the egg, vanilla, and the zest and juice of the lemon. Mix well, then beat in the milk.
¼ cup unsalted butter, ¾ cup granulated sugar, 1 large egg, 1 teaspoon pure vanilla extract, 1 lemon, ½ cup milk
In a separate medium-sized bowl, combine the flour, baking powder, and salt.
2 cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon kosher salt
Gently mix the wet and dry ingredients until just combined.
Gently fold in the raspberries.
1½ cups fresh raspberries
Fill each well of the prepared muffin tin ¾ full with batter. Set aside.
Make the streusel topping: In a small bowl, combine brown sugar, flour, and lemon zest.
½ cup brown sugar, ⅓ cup all-purpose flour, 2 teaspoons grated lemon zest
Cut in the butter with a fork or pastry cutter until crumbly.
¼ cup unsalted butter
Sprinkle the streusel evenly over each muffin.
Bake 25 minutes, or until a toothpick inserted in muffin comes out clean.
Cool slightly and serve!