01 - Preheat oven to 350°F. Grease an 8-inch round cake pan and line with parchment paper.
02 - Whisk together flour, granulated sugar, baking powder, salt, cinnamon, and nutmeg in a large bowl.
03 - Rub cold cubed butter into dry ingredients using fingertips until mixture resembles coarse crumbs.
04 - Add sliced apples to flour mixture and toss to coat evenly.
05 - Whisk eggs, milk, and vanilla extract in a separate bowl.
06 - Pour egg mixture over apple mixture and gently fold until just combined.
07 - Transfer batter to prepared pan, spreading evenly. Sprinkle turbinado sugar over top.
08 - Bake for 45-50 minutes until skewer comes out clean and top is golden. Cool 15 minutes in pan.
09 - Heat milk in saucepan over medium heat until just steaming, not boiling.
10 - Whisk egg yolks, sugar, and cornstarch until smooth.
11 - Gradually whisk hot milk into egg mixture, then return to saucepan.
12 - Cook over low heat, stirring constantly, until thickened and coats back of spoon, 5-7 minutes. Do not boil. Stir in vanilla.
13 - Serve warm cake slices with generous custard.