1. Preheat your oven to 350°F. In a mixer, cream the butter on medium-high speed until it's soft, smooth and light.
2. Add both of the sugars to the creamed butter and mix until they are fully incorporated, and lighter in color. Stirring vigorously, add the egg to the mixture and beat until the batter looks uniform.In a separate bowl, stir together the flour, cornstarch, baking soda, salt and nutmeg. Then slowly fold into the batter until just combined. Then add in diced apples, and gently fold them by hand into the batter until well-distributed.Form balls of dough (I used a cookie scooper) and plop them in cinnamon sugar, swirling to coat. On a greased or parchment lined cookie sheet, place the dough balls 3 inches apart. Use your fingers to press and flatten the dough slightly and sprinkle an extra pinch of cinnamon sugar on the flattened surface.Set the first timer for 7 minutes. Turn the pan around and set a second timer for 7 minutes. If you are using a greased pan they might be done at this point, or need another minute. If you’re using parchment, cookies will take 2-3 minutes more. You are looking for golden edges and a blonde middle.Cool on a baking sheet and enjoy. Snickerdoodles will keep in an airtight container, at room temperature for 3 days, but they are best eaten the day they’re made.