Preheat Oven: Preheat the oven to 350°F.
Combine Dry Ingredients: In a large mixing bowl, combine sugar, flour, baking soda, baking powder, salt, and cinnamon.
Cut in Butter: Add cold butter pieces and mix with an electric mixer until fully incorporated and the mixture resembles fine sand.
Add Wet Ingredients: Slowly mix in applesauce and milk. Add eggs one at a time, followed by vanilla. Mix just until combined. Do not overmix.
Prepare Pan: Line the bottom of a 10-inch springform pan with parchment paper. Butter the bottom and sides, then coat with approximately one tablespoon each of sugar and flour. Discard excess.
Make Streusel: Combine butter, sugar, flour, cinnamon, and salt in a small bowl. Mix until pea-sized crumbs form.
Prepare Apples: Wash, peel, and thinly slice apples.
Layer Cake: Spread approximately one cup of batter into the prepared pan. Cover with a thin layer of apple slices.
Add Remaining Batter: Spread remaining batter over the apples. Arrange another layer of apple slices on top.
Add Streusel: Sprinkle half of the streusel over the apples. Arrange remaining apple slices on top and finish with the remaining streusel.
Bake: Bake for approximately 40–45 minutes, or until a toothpick inserted into the center of the cake comes out clean or with a few moist crumbs.
Cool: Let the cake cool in the pan for 10–15 minutes before removing the springform ring. Cool completely before serving.