Preheat oven to 400°F and line a muffin pan with paper liners or grease with a little butter or nonstick spray.
Add flour, baking powder, baking soda, salt, ground cinnamon, and brown sugar to a large mixing bowl and combine.
In a separate bowl, stir together melted butter, discard, eggs, sour cream, and vanilla
Pour wet ingredients into dry ingredients and combine. Then fold in the chopped apples.
Mix up your streusel topping by adding the flour and brown sugar to a small bowl and then mixing in the melted butter with a fork until you have a crumbly mixture.
Fill muffin cups 3/4 way full with muffin batter and then sprinkle on streusel topping. Lightly press the topping onto the muffin mix.
Bake at 400°F for 5 minutes and then drop the temperature to 350° without opening the oven door for about 15 more minutes.
Cool on a wire rack and enjoy.