Pre-heat the oven to 425℉ and line the muffin tin with cupcake liners.
In a medium mixing bowl, whisk together the melted butter, sugar, light brown sugar, and vegetable oil until combined.
Mix in the vanilla extract.
Next, add one egg at a time and whisking until combined with each egg addition.
In a separate mixing bowl, add the gluten free flour, salt, baking powder, and cinnamon. Whisk together until all the dry ingredients are evenly distributed.
Add the diced apple pieces to the flour mixture and toss together.
Next, using a rubber spatula, fold in half of the flour apple mixture into the batter just until the flour is almost completely mixed in, some streaks of flour should remain.
Next, fold in all of the sour cream until the sour cream is almost completely mixed in.
Finally, add the last half of the flour apple mixture to the batter and gentle fold in just until the flour is combined. DO NOT OVERMIX!
Let the batter rest for 30 minutes in the bowl before scooping.
After the batter has rested, using a cookie scoop, gently scoop the muffin batter into the pan so that each muffin is completely full.
Sprinkle the streusel topping on top of the muffins before placing in the oven.
Place the muffins in the oven and bake at 425℉ for 5 minutes.
Once the muffins have been in the oven for the first 5 minutes, lower the oven temperature to 350℉ and DO NOT open the oven door.
Bake at 350℉ for another 25 minutes until the muffins are slightly golden brown, or a toothpick test comes out clean.