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

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

In another bowl, mix the buttermilk, vegetable oil, 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, combine the flour, brown sugar, and cinnamon.

Add the melted butter and mix until crumbly.

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

Sprinkle the streusel topping evenly over the batter in each cup.

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

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