Preheat oven to 375°F (190°C). Line or grease a 12-cup muffin tin.
Make the streusel: mix flour, brown sugar, and cinnamon. Rub in softened butter with fingertips until crumbs form. Refrigerate while preparing batter.
Whisk flour, sugar, baking powder, salt, and cinnamon in a large bowl. Make a well in the center.
In another bowl, whisk melted butter, eggs, milk, and vanilla. Pour into dry ingredients and fold until just combined. Do not overmix.
Divide batter evenly into 12 muffin cups, filling about ¾ full.
Add chilled streusel generously on top and lightly press to help it stick.
Bake for 18–22 minutes until golden and a toothpick comes out clean.
Cool in pan for 5 minutes, then transfer to a wire rack.