Preheat oven to 350℉ and line muffin pan with 18 paper liner. With a mixer, cream together sugar, eggs, oil, and vanilla. The mixture should be a pale yellow. In a separate bowl, whisk together flour, baking soda, salt, and ground cinnamon. Add dry ingredients to creamed mixture and mix until combined. The batter will be very thick, almost like the texture of cookie dough. Mix in the diced apples. The dough will loosen up a bit when the apples are mixed in. Fill paper liners almost to the top, about ¾ of the way full. Sprinkle each muffin top generously with brown sugar. Bake for 20-24 minutes.