Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, whisk together 2 cups of flour, granulated sugar, baking powder, and salt.
In another bowl, combine melted butter, egg, milk, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until just combined.
Gently fold in the chocolate chips.
In a separate small bowl, prepare the crumble topping by mixing together 1/2 cup flour, brown sugar, cold cubed butter, and cinnamon.
Use a fork or your fingers to combine until the mixture resembles coarse crumbs.
Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.
Sprinkle the crumble topping evenly over each muffin.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.