Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt.

In a separate bowl, combine the egg, milk, melted butter, and vanilla extract, whisking until smooth.

Gently fold the wet ingredients into the dry mixture using a spatula, careful not to overmix.

Prepare the streusel by mixing brown sugar, flour, cinnamon, and softened butter until crumbly.

Spoon the muffin batter into muffin cups, filling them about three-quarters full, then sprinkle with streusel.

Bake for 20-25 minutes until golden brown and a toothpick inserted comes out clean.

Cool for 5 minutes in the pan before transferring to a wire rack.