Heat oven to 375°F. Line a 12-cup muffin pan with liners.
Stir streusel flour, brown sugar, cinnamon, and salt. Drizzle in melted butter and toss into clumps.
Whisk batter flour, granulated sugar, baking powder, baking soda, salt, and cinnamon in a large bowl.
Whisk melted butter, eggs, sour cream, milk, and vanilla in a separate bowl until smooth.
Fold wet ingredients into dry just until combined. Do not overmix.
Spoon batter to fill cups halfway. Add a spoonful of streusel, then top with remaining batter.
Sprinkle more streusel over each muffin, pressing lightly so it adheres.
Bake 16–18 minutes, until a toothpick comes out clean or with a few moist crumbs.
Cool in pan 5 minutes, then move to a rack.
Whisk powdered sugar, milk, and vanilla until smooth. Drizzle over warm muffins and serve.
Notes
Variation: Stir 1/2 cup chopped toasted pecans into the streusel or swirl 1/3 cup cinnamon sugar through the batter. Storage: Keep airtight at room temperature up to 3 days, or freeze up to 2 months and rewarm before glazing.This recipe is an original creation inspired by classic Coffee Cake Muffins Made Easy flavors. All ingredient ratios and instructions are independently developed.