Heat oven to 400 degrees with oven rack in middle. Lightly spray 12 cups in a standard muffin pan with nonstick cooking spray.
Streusel Topping: combine flour, brown sugar, and cinnamon in a small bowl. Cut in butter with a pastry blender until mixture resembles coarse crumbs with some pea-sized pieces.
Muffins: Combine flour, sugar, brown sugar, baking powder, cinnamon, and salt in a large bowl. Make a well in center of the flour by pushing ingredients out toward sides of bowl.
Combine milk, melted butter, vanilla, and egg in a medium bowl. Pour milk mixture into the flour mixture, and stir only until the flour is moistened even though the batter is not smooth. Scrape down sides of bowl.
Divide batter into prepared muffin cups, filling them about 2/3 full. Sprinkle about 1 tablespoon of streusel over each muffin.
Bake 18 to 20 minutes or until golden brown and a toothpick inserted in the center of a couple of muffins comes out dry. Cool slightly and remove from the pan.
Run a small metal spatula around edge of each muffin to loosen it, and lift gently from the pan. Serve muffins warm with butter or jam.
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