Preheat your oven to 375°F (190°C) and prepare a muffin tin with paper liners or a light greasing.

In a large mixing bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon until well combined.

In another bowl, mix the melted butter, milk, eggs, and vanilla extract until thoroughly blended.

Pour the wet ingredients into the dry mixture. Stir gently until just combined—remember, don’t overmix!

For the streusel topping, combine the flour, brown sugar, and cinnamon in a small bowl. Add the cold butter and mix with your hands or a pastry cutter until you achieve coarse crumbs.

Fill each muffin cup about 2/3 full with the batter, then sprinkle the streusel topping generously over each.

Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.

Allow to cool in the pan for about 5 minutes before transferring muffins to a wire rack to cool completely.