Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease it lightly.

In a large bowl, mix the flour, granulated sugar, baking powder, salt, and cinnamon.

In a separate bowl, combine the melted butter, milk, eggs, and vanilla extract. Whisk together until well combined.

Pour the wet ingredients into the dry ingredients and mix gently until just combined. Be careful not to overmix.

In a small bowl, prepare the streusel topping by mixing the flour, brown sugar, and cinnamon. Add in the cold butter pieces and mix with your fingers or a pastry cutter until you have coarse crumbs.

Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.

Sprinkle the streusel mixture evenly over the top of the muffin batter.

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

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