Preheat your oven to 375°F. Line a 12 well muffin tin with liners, or grease well.
Whisk together all your dry ingredients in a large mixing bowl.
Combine your egg, butter and milk in a measuring cup .Pour the wet mixture into the dry ingredients, combining thoroughly.
Add in your prepared apples and mix in.
Divide the batter evenly between 12 muffin cups.
For the streusel, whisk together the brown sugar, flour and cinnamon. Grate the cold butter into the mixture, then use a fork to combine and make it as crumbly as possible
Top the muffins with the streusel mixture, dividing it evenly between the 12 muffins.
Bake at 375 for 15-18 minutes, until the muffins are golden brown and a toothpick inserted comes out clean,
Remove and cool on a baking rack, then store in a closed container at room temperature for 3-4 days.