Preheat Oven: Set oven to 350°F.
Prepare Pan: Line an 8" or 9" springform pan with parchment paper and spray with oil. Ensure the 8" pan is deep enough to hold all the batter.
Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, salt, and cinnamon. Set aside.
Cream Butter and Sugar: Beat butter with an electric mixer for 1 minute until creamy. Add the sugar and beat for 2 more minutes until light and fluffy, scraping the bowl as needed.
Add Eggs and Vanilla: Add the eggs one at a time, mixing well after each addition. Mix in the vanilla.
Add Buttermilk and Dry Ingredients: Mix in the buttermilk. Add the flour mixture and mix on low or fold with a spatula until just combined.
Assemble Cake: Spread ⅔ of the batter in the prepared pan. Layer with half the apples. Top with the remaining batter and smooth out. Add the rest of the apples and sprinkle with the streusel crumb.
Bake: Bake for 55-65 minutes. If the crumb browns too quickly and the center isn’t done, cover with foil and continue baking until a toothpick comes out clean. Let the cake cool completely.