Make the crumb topping. Toss the sugar, flour, and cinnamon together in a small bowl. Cut in the butter chunks until large crumbs form. Place the bowl in the fridge while you make the batter.
Line muffin cups with muffin liners or parchment paper squares (or spray the muffin wells with nonstick spray.
In a large bowl, stir together all the dry ingredients, including the sugar.
In a large liquid measuring cup, add the buttermilk (or milk), oil, eggs, and vanilla. Whisk to combine.
Make a well in the center of the dry ingredients and pour in the wet ingredients, followed by the diced apples.
Stir everything to combine. Don't overmix.
Using a large scoop or spoon, fill the muffin wells all the way to the top with batter. If making jumbo muffins, this will be about one cup EACH! (I use two large scoops to fill one jumbo well).
Sprinkle the tops with crumbs, pressing the crumbs lightly to adhere.
Bake at 475° F for 5 minutes. Turn down the oven temperature to 375° F and continue to bake for another 30 minutes for jumbo muffins or 20-22 minutes for standard-sized muffins, or until a wooden skewer or toothpick inserted into the center comes out clean.
Cool in the pan on a wire rack for 5 minutes. Remove the muffins from the pan and cool for an additional 10-15 minutes before serving warm or at room temperature.
Muffins will last for a few days at room temperature in a covered container, or may be frozen for up to 3 months by wrapping individually in plastic wrap and storing in a large ziptop bag.