01 - Preheat oven to 350°F. Grease and line a 9-inch round cake pan.
02 - In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
03 - Add the cold, cubed butter to the dry mixture. Using your fingertips, rub the butter into the flour until the mixture resembles coarse crumbs.
04 - Stir the granulated sugar and thinly sliced apples into the flour mixture, ensuring the apple slices are well coated.
05 - In a separate bowl, whisk together the eggs, whole milk, and vanilla extract.
06 - Pour the wet ingredients into the dry mixture. Mix gently until just combined, being careful not to overmix.
07 - Pour the cake batter into the prepared 9-inch cake pan and smooth the top evenly.
08 - Bake for 45–50 minutes, or until a wooden skewer inserted into the center of the cake comes out clean. Allow the cake to cool in the pan for 10 minutes before carefully removing it.
09 - While the cake is baking, begin the custard sauce. In a saucepan, heat the whole milk over medium heat until it just begins to simmer.
10 - In a separate bowl, whisk together the egg yolks, granulated sugar, and cornstarch until the mixture is pale and thick.
11 - Gradually pour a small amount of the hot milk into the egg yolk mixture, whisking constantly to temper the yolks. Then, slowly pour the entire tempered mixture back into the saucepan with the remaining hot milk.
12 - Cook the custard over low heat, stirring constantly with a wooden spoon or spatula, until it thickens sufficiently to coat the back of the spoon. Do not allow the custard to boil.
13 - Remove the saucepan from the heat and stir in the vanilla extract. For an extra smooth custard, strain it through a fine-mesh sieve if desired.
14 - Serve warm slices of the Irish Apple Cake with generous spoonfuls of the homemade custard sauce.