Apple Cinnamon Streusel Muffins

Preheat your oven to 375°F and line a 12-cup muffin tin with paper liners or grease the cups.
Make the streusel by combining 1/2 cup flour, brown sugar, and 1 teaspoon cinnamon in a small bowl. Cut in the cold cubed butter with a pastry blender or two forks until the mixture resembles coarse crumbs, then set aside.
Whisk together the 2 cups flour, granulated sugar, baking powder, salt, and 1 teaspoon cinnamon in a large bowl until thoroughly combined.
In a separate medium bowl, whisk the egg, milk, melted butter, and vanilla extract until smooth. Pour the wet ingredients into the dry ingredients and stir with a rubber spatula until just combined; the batter will be thick and a few lumps are okay. Gently fold in the chopped apple.
Divide the batter evenly among the prepared muffin cups, filling each about 2/3 full. Generously sprinkle the streusel topping over each portion of batter, gently pressing it down slightly so it adheres.
Bake for 20 to 25 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are golden brown. Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.