Preheat the oven to 350°F (175°C) and grease a muffin tin.

In a large bowl, whisk together flour, sugar, baking powder, baking soda, cinnamon, and salt.

In another bowl, mix melted butter, buttermilk, and eggs until well combined.

Pour the wet ingredients into the dry ingredients and stir until just combined. Fold in the diced apples.

In a small bowl, mix together flour, brown sugar, and cinnamon.

Add the cold butter and mix with a fork or pastry cutter until crumbly.

Spoon the muffin batter into the prepared muffin tin, filling each cup about 2/3 full.

Sprinkle the streusel topping evenly over each muffin.

Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.