Preheat your oven to 425 degrees.
Spray a standard muffin tin lightly with baking spray and line with paper muffin cups.
In a medium to large mixing bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
Add the peeled, diced apples to the dry ingredients. Stir to combine. This helps the apples to disperse evenly through the muffins and not sink to the bottom. Make sure the apples are diced small, about one centimeter squared or smaller. Big chunks of apple won't cook evenly.
In a small mixing bowl, whisk together the butter and the sugar.
Add the eggs, milk, sour cream and vanilla to the butter and sugar mixture. Whisk to combine.
Pour the wet ingredients into the dry mixture. With a rubber spatula, fold the wet and dry ingredients together until you have a uniform batter. Do not overmix.
Fill each muffin cup about ¾ full. You can use a #20 scoop if you wish for more uniform muffins.
Sprinkle a layer of turbinado sugar, if using, on top of each muffin. This will form a delightful crunchy top on the muffins as they bake.
Bake at 425 degrees Fahrenheit for 5 minutes, then lower the temperature of the oven to 375 degrees Fahrenheit and bake for an additional 10 minutes.
Muffins are done when they spring back lightly when pressed.
Cool on a cooling rack and enjoy!
