Preheat the oven to 375℉. Line a muffin tin with 12 liners and set aside. You can use regular liners or tulip liners for a taller bake like I did. If you use regular liners, you will have 15-17 muffins. Please note that tulip liners do require a longer bake because there is more batter in the muffins. Chop the apples (skin on or off, up to you) into small pieces and set aside. Melt the butter and allow it to cool as you prep the rest of your ingredients.
2 cups apples, 1/4 cup butter
In a medium-sized bowl, add in flour, baking soda, baking powder, cinnamon, cornstarch, and salt. Mix together and set aside.
2 cups all purpose flour, 2 tsps baking powder, 1 tsp baking soda, 1/4 tsp salt, 1 1/2 tsps cinnamon, 1 tbsp cornstarch
In a large bowl, add brown sugar, melted and slightly cooled butter, oil, eggs, vanilla, and sour cream. Whisk until well combined.
1 cup light brown sugar, 3/4 cups sour cream, 1/4 cup oil, 2 large eggs, 1 tsp vanilla, 1/4 cup butter
Pour the dry ingredients into the wet and using a spatula, gently fold to create a soft muffin batter.
Mix in the apples until just combined.
2 cups apples
Evenly divide the batter between 12 liners or more depending what kind of liners you're using, they should be almost or all the way full. Using a cookie scooper helps big time to minimize the mess. Sprinkle streusel on top.
Bake the muffins at 375°F for 18-24 minutes The muffins are ready when a toothpick inserted comes out clean.