Preheat your oven to 375°F (190°C) and line a 12-cup muffin pan with paper liners.
Mix dry ingredients: In a large bowl, whisk together flour, sugar, baking powder, cinnamon, and salt.
Prepare wet ingredients: In a separate bowl, whisk together egg, milk, melted butter, and vanilla extract until smooth.
Combine wet and dry ingredients: Pour the wet mixture into the dry ingredients and stir until just combined. Do not overmix.
Fold in apples: Gently fold in chopped apples, ensuring they’re evenly distributed throughout the batter.
Make the streusel: In a small bowl, mix flour, brown sugar, and cinnamon. Add cold butter and use a pastry blender or your fingers to cut the butter into the dry ingredients until crumbly.
Fill muffin pan: Divide the batter evenly among the muffin cups and top each with a generous sprinkle of streusel.
Bake for 18-22 minutes or until a skewer inserted into the center of a muffin comes out clean.
Cool and serve: Allow the muffins to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.