01 - Preheat oven to 350°F. Grease and flour a 9-inch round cake pan.
02 - In a medium bowl, whisk together flour, baking powder, cinnamon, nutmeg, and salt.
03 - In a large bowl, cream the butter and sugar until light and fluffy. Beat in the eggs one at a time, then mix in the vanilla.
04 - Alternately add the flour mixture and milk to the butter mixture, beginning and ending with flour. Mix until just combined.
05 - Gently fold in the apple slices. Spread the batter evenly into the prepared pan.
06 - Mix 2 tablespoons sugar with ½ teaspoon cinnamon and sprinkle over the top of the cake.
07 - Bake for 50 to 55 minutes, or until a toothpick inserted in the center comes out clean. Cool in the pan for 10 minutes, then turn out onto a wire rack.
08 - While the cake cools, prepare the custard. In a saucepan, heat the milk until just simmering.
09 - In a bowl, whisk egg yolks, sugar, and cornstarch until smooth.
10 - Gradually whisk the hot milk into the yolk mixture, then return to the saucepan.
11 - Cook over medium-low heat, stirring constantly, until the custard thickens for about 5 minutes. Do not boil.
12 - Remove from heat and stir in vanilla extract. Serve the cake warm or at room temperature with the warm vanilla custard poured over each slice.