1Position a rack in the middle of the oven and preheat to 400°F. Line 12 standard muffin cups with paper liners, or grease with cooking spray.
2To make the streusel, in a bowl, stir together the melted butter and vanilla. In another bowl, combine the flour, sugar, and salt and stir together with a fork. Add the butter mixture and stir until evenly blended and crumbly. Use right away or refrigerate, covered, until ready to use.
3In a bowl, sift together the flour, baking powder, baking soda, and salt. In the bowl of a mixer fitted with the paddle attachment, beat the butter, sugar, and lemon zest on medium-high speed until fluffy, about 1 minute. Beat in the eggs, one at a time, then the vanilla, until well combined, scraping down the sides of the bowl with a rubber spatula. Add the sour cream and beat on low speed. Add the dry ingredients and, using the rubber spatula, stir just until evenly moistened. Gently fold in the berries. The batter will be very thick.
4Using an ice cream scoop, heap the batter into the prepared muffin cups, filling them full. Sprinkle the streusel over the tops of the muffins, dividing it evenly and pressing it down to stick to the batter. Bake until the muffins are golden brown and a toothpick inserted into the center of one comes out clean, about 15 minutes. Let the muffins cool in the pan on a wire rack for 5 minutes, then turn out onto the rack.