Preheat the Oven:

Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease it lightly.

Prepare the Muffin Batter:

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

In a separate bowl, whisk together the vegetable oil, egg, milk, and vanilla extract. Add the wet ingredients to the dry ingredients and mix until just combined. Fold in the diced apples.

Make the Crumb Topping:

In a small bowl, combine the flour, granulated sugar, cubed butter, and ground cinnamon. Use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs.

Fill the Muffin Tin:

Divide the muffin batter evenly among the prepared muffin cups, filling each about 2/3 full. Sprinkle the crumb topping generously over each muffin.

Bake the Muffins:

Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean. Allow the muffins to cool in the tin for a few minutes before transferring them to a cooling rack to cool completely.