Preheat the oven to 350°F (175°C) and prepare a muffin pan with paper liners.

Cream together the sugar, eggs, vegetable oil, and vanilla extract until pale and fluffy.

In a separate bowl, whisk together flour, baking soda, salt, and cinnamon.

Gradually add the dry ingredients to the wet mixture, folding gently.

Fold in the diced apples until evenly distributed.

Divide the batter among the muffin liners, filling each about 3/4 full.

Sprinkle brown sugar on top of each muffin.

Bake for 20-24 minutes until golden brown and a toothpick comes out clean.

Allow muffins to cool in the pan for about 5 minutes before transferring to a cooling rack.