Blueberry Sour Cream Muffins

MAKE THE TOPPING: Preheat the oven to 375 degrees F. In a medium bowl, combine the brown sugar, flour, butter and cinnamon. Rub with your fingers to form coarse crumbs. Refrigerate until ready to use.
MAKE THE MUFFINS: Butter a 12-cup muffin pan. In a medium bowl, whisk the 1 ½ cups of flour with the baking powder, baking soda and salt. In a large bowl, using a hand-held mixer, beat the egg at medium speed until frothy. Add the sugar and melted butter and beat until pale yellow, 1 minute. Beat in the sour cream, vanilla and lemon zest until blended. Add the dry ingredients and beat at low speed until almost blended. In a bowl, toss the blueberries with the remaining 1 tablespoon of flour. Using a rubber spatula, fold the blueberries into the batter.
Fill the muffin cups three-fourths full of batter; sprinkle with the topping. Bake for 25 minutes, or until a cake tester comes out clean. Remove the muffins from the pan; let cool on a rack.