Prepare the Baking Pan: Preheat the oven to 350°F (175°C). Line the bottom of a 9-inch springform pan with a round piece of parchment paper, then coat the pan with nonstick cooking spray. Set aside.
Make the Crumble Topping: In a mixing bowl, whisk together the flour, rolled oats, granulated sugar, and salt. Add the cold, cubed butter and work it into the dry ingredients using a pastry cutter or two forks until the mixture resembles coarse sand. Place the crumble mixture in the refrigerator while preparing the cake batter. Combine the Dry Cake Ingredients: In a large bowl, whisk together the flour, baking powder, ground cinnamon, and salt until evenly combined. Set aside.
Cream the Butter and Sugar: In a separate large bowl, beat the room-temperature butter and granulated sugar with a stand mixer or hand mixer for about 1 minute, until the mixture looks creamy.
Add the Eggs and Flavorings: Beat in the eggs one at a time, mixing after each addition. Add the vanilla extract and room-temperature milk, then mix until incorporated. Scrape down the sides and bottom of the bowl as needed to ensure the ingredients are evenly combined. Finish the Cake Batter: Add the prepared dry ingredients to the butter mixture. Mix just until the flour disappears and the ingredients are combined. The batter will be thick, which is expected.
Assemble the Cake: Transfer the thick batter to the prepared springform pan and spread it into an even layer. Arrange the peeled, thinly sliced Granny Smith apples over the batter in a single layer. Remove the crumble topping from the refrigerator and distribute it evenly over the apples. Bake the Cake: Place the pan in the preheated 350°F (175°C) oven and bake for 55–60 minutes, until the cake is baked through and the crumble topping is golden.
Cool and Release the Cake: Allow the cake to cool in the pan for approximately 15 minutes. Carefully run a knife around the inside edge to loosen it, then remove the sides of the springform pan. Let the cake cool completely before serving.
Whisk the Custard Ingredients: In a medium bowl, whisk together the egg yolks, granulated sugar, and vanilla extract for about 2 minutes, until the mixture becomes pale. Set aside.
Heat the Milk: Pour the whole milk into a medium saucepan and heat over medium heat until scalding hot. Look for small bubbles beginning to form around the edges of the pan, indicating that the milk is just starting to simmer.
Temper the Egg Yolks: Slowly pour approximately ½ cup of the hot milk into the egg yolk mixture while whisking constantly. This gradually warms the yolks. Pour the tempered egg mixture back into the saucepan with the remaining hot milk, continuing to whisk as you combine everything. Cook the Custard Sauce: Cook the mixture over medium heat for about 4 minutes, stirring constantly to prevent the custard from scorching. It is ready when it thickens enough to coat the back of a spoon.
Transfer the Custard: Remove the thickened custard from the heat and transfer it to a sauce boat or serving bowl.
Slice and Serve: Cut the completely cooled apple cake into 10 slices. Serve each slice with a drizzle of the warm or cooled vanilla custard sauce, as preferred.
