01 - Preheat oven to 350°F. Grease an 8-inch round cake pan and line with parchment paper.
02 - Whisk flour, granulated sugar, baking powder, salt, cinnamon, and nutmeg in a large bowl.
03 - Rub cold cubed butter into dry mixture with fingertips until coarse crumbs form.
04 - Add sliced apples to flour mixture and toss until evenly coated.
05 - Whisk eggs, milk, and vanilla extract in a separate bowl until blended.
06 - Pour wet mixture over apple mixture. Fold gently until just combined.
07 - Spread batter evenly in prepared pan. Sprinkle turbinado sugar over surface.
08 - Bake 45-50 minutes until skewer comes out clean and top is golden. Cool 15 minutes in pan, then transfer to wire rack.
09 - Heat milk in medium saucepan over medium heat until steaming, not boiling.
10 - Whisk egg yolks, sugar, and cornstarch until smooth.
11 - Gradually whisk hot milk into egg mixture. Return to saucepan and cook over low heat, stirring constantly, until thickened, 5-7 minutes. Remove from heat and stir in vanilla.
12 - Serve warm cake slices with custard poured over top.