Preheat your oven to 350°F (175°C). It’s crucial to have the proper temperature so that your cake bakes evenly.
Line a 9-inch springform pan or a 9-inch cake pan with parchment paper. Use some of the cooking spray or softened butter to grease the pan, ensuring the cake comes out easily after baking.
In a medium-sized bowl, mix together the ¾ cup all-purpose flour, ¼ cup rolled oats, ½ cup sugar, and ¼ teaspoon salt. Use a knife to cut the batter into cubes and rub it into the dry ingredients until it resembles coarse crumbs. Place this mixture in the refrigerator while you prepare the cake batter.
In a stand mixer, cream the 8 tablespoons of room temperature butter with the remaining ½ cup sugar until light and fluffy, about 4 to 5 minutes. This step is vital for incorporating air into the batter, which helps it rise.
Mix in the 2 teaspoons vanilla extract, and then beat in the 2 eggs, one at a time, ensuring each is fully incorporated before adding the next.
In a separate bowl, whisk together the remaining 1¼ cups of all-purpose flour, 1 teaspoon baking powder, 1 teaspoon cinnamon, and ¼ teaspoon salt. With the mixer running, slowly add this flour mixture to the wet mixture, blending just until combined. The batter will be thick.
Mix in the 3 tablespoons of whole milk until the batter is smooth. This will help loosen the batter slightly, making it easier to spread in the pan.
Once the batter is ready, transfer it into the prepared pan. Layer the 3 Granny Smith apples over the top, arranging them nicely for an appealing presentation. Then, sprinkle the reserved crumble mixture on top.
Bake for 55 minutes, or until the top is lightly browned, and a toothpick inserted in the center comes out clean. Allow it to cool slightly in the pan.
Meanwhile, prepare the custard. In a medium saucepan, heat the 1½ cups whole milk over medium heat until just simmering, being careful not to let it boil. In a separate bowl, whisk together the 6 egg yolks with 6 tablespoons of sugar until well combined.
While whisking constantly, slowly pour about half of the warmed milk into the egg mixture. This will temper the eggs. Then transfer the mixture back to the saucepan and cook over medium heat, stirring with a wooden spoon constantly until just thickened. Do not overcook!
Once thickened, stir in the 2 teaspoons vanilla extract. If the custard is lumpy, strain it through a fine-mesh sieve for a silky texture.
Slice the cooled cake and serve it drizzled with the warm custard. Enjoy every bite of this comforting treat!
