Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease each cup lightly with butter.

In a small bowl, combine streusel flour, brown sugar, and cinnamon. Cut in cold butter with a pastry cutter or two knives until mixture clumps into pea-size bits. Set aside in the fridge to firm up.

In a large bowl, whisk together flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon for the batter.

In a separate bowl, whisk milk, melted butter, eggs, vanilla, and sour cream until smooth.

Pour wet into dry ingredients. Stir just until no large streaks of flour remain—overmixing can lead to dense muffins. A few small lumps are perfect for a light crumb.

Divide batter evenly among 12 cups (about ⅓ cup each). You can use a medium cookie scoop for uniform muffins.

Sprinkle chilled streusel generously over each muffin—aim for about 1 dessert-spoon per cup so you get a crunchy crown.

Bake for 18–22 minutes, rotating the pan halfway through. When the tops are golden and a toothpick inserted in the center comes out clean (with a few moist crumbs), they’re done.

Let muffins rest in the tin for 5 minutes, then transfer to a wire rack. Serve warm or at room temperature—either way, the aroma is irresistible.