Preheat + dry ingredients: Preheat the oven to 425 degrees Fahrenheit. In a large bowl mix the dry ingredients - flour, sugar, cinnamon, baking powder, baking soda, and salt. Use a whisk to mix everything well.
Mix wet ingredients: In a medium mixing bowl place the eggs, oil (or melted butter), milk, and vanilla. Whisk together until it is well combined.
Fold everything together: Chop the apple into small bite-sized pieces. Add the apples, chocolate chips, and flour mixture to the wet mixture. Use a spatula to stir the batter together, about 12-15 folds, just until everything is incorporated. Do not over-stir, this is key for well-risen fluffy muffins.
Scoop & bake: Add about ⅓ cup of batter to fill each muffin cup. Place the muffin tin in the preheated oven to bake for 8 minutes, then turn the oven down to 375 degrees Fahrenheit and bake for another 7 minutes. Remove muffins from the oven when they spring back from the touch and when the center does not look gooey.