Preheat your oven to 350°F (175°C).
In a large bowl, mix together the flour, sugar, baking powder, baking soda, cinnamon, and salt.
In another bowl, whisk together the melted butter, eggs, and buttermilk until well combined.
Pour the wet ingredients into the dry ingredients and mix until just combined.
Fold in the chopped apples gently into the batter.
In a small bowl, combine the brown sugar, rolled oats, additional flour, and cinnamon to make the streusel topping.
Divide the muffin batter evenly into a greased or lined muffin tin.
Sprinkle a generous amount of streusel topping over each muffin.
Bake for 18-22 minutes or until a toothpick inserted into the center comes out clean.
Allow muffins to cool in the pan for a few minutes, then transfer them to a wire rack.
