Preheat your oven to 350°F. Grease a 9-inch springform pan generously, then line the bottom with parchment paper.

In a large bowl, whisk together the flour, baking powder, salt, and cinnamon.

Add the cold, cubed butter to your dry mixture. Using your fingers or a pastry cutter, work the butter in until the mixture looks like coarse crumbs.

In a separate bowl, whisk the eggs, milk, and vanilla extract together until smooth. Pour this mixture into your dry ingredients. Stir gently, just until everything comes together.

Toss your diced apples in a spoonful of the dry mixture before adding them to the batter.

In a small bowl, combine the flour, brown sugar, and cinnamon for the topping. Pour in the melted butter and use a fork to cut everything together until it looks like coarse crumbs.

Pour the batter into your prepared pan and smooth the top. Sprinkle the crumble topping evenly over the surface, then add the turbinado sugar if using.

Bake for 50 to 55 minutes. Check doneness with a toothpick inserted into the center.

Let the cake cool in the pan for at least 20 minutes before releasing the springform sides.