Preheat oven to 350ºF. Spray a 9-inch round cake pan (or springform pan) with cooking spray and set aside. If you'd like to completely remove the whole cake from the pan to serve, line the cake pan with parchment paper before adding the cake batter.)

In the bowl of a stand mixer (or with a large bowl and hand mixer), whip together the butter and 1 cup of the sugar (the other ¼ will go on top) until pale and fluffy, about five minutes. ½ cup salted butter, 1 ¼ cup granulated sugar

Add the eggs, one at a time, and beat well after each addition. Beat in the vanilla. 2 large eggs, 1 teaspoon vanilla extract

Add half of the flour, 1 teaspoon cinnamon, baking powder and salt to the butter mixture. Mix gently. Add the sour cream, mixing until barely combined, then add the remaining flour. Mix just until combined after each addition. 1 ⅓ cup all-purpose flour, 1 ½ teaspoons ground cinnamon, 1 ¼ teaspoons baking powder, ¼ teaspoons salt, ½ cup full-fat sour cream

Gently fold in the diced apples to the cake batter using a rubber spatula. Spread the batter in the prepared baking pan. Combine the remaining ¼ cup sugar and ½ teaspoon of ground cinnamon in a bowl. Sprinkle evenly over the top of the cake batter. 2 cups finely diced Granny Smith apples

Bake 35-40 minutes, until the top is crackly and lightly browned and the center still jiggles just a bit. A toothpick inserted in the center will have moist crumbs.

Let the cake cool completely in the pan and then serve.