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

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

In a separate bowl, whisk together the melted butter, milk, eggs, and vanilla extract until well blended.

Pour the wet ingredients into the dry ingredients. Gently mix together until just combined, being careful not to overmix.

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

Spoon the muffin batter into the prepared muffin tin, filling each cup about two-thirds full.

Evenly sprinkle the streusel mixture over the top of each muffin.

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

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