01 - Preheat your oven to 350°F. Thoroughly grease and line a 9-inch round cake pan with parchment paper.
02 - In a spacious mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg until well combined.
03 - Add the cold, cubed unsalted butter to the dry ingredient mixture. Using your fingertips, rub the butter into the flour until the mixture resembles coarse crumbs.
04 - Stir the granulated sugar into the crumb mixture, then add the thinly sliced Granny Smith apples. Ensure all apple slices are thoroughly coated with the flour mixture.
05 - In a separate bowl, whisk together the large eggs, whole milk, and vanilla extract until smooth.
06 - Pour the wet ingredients into the dry ingredients. Mix gently with a spatula until just combined; avoid overmixing.
07 - Transfer the cake batter into the prepared 9-inch cake pan and gently smooth the top surface. Bake for 45 to 50 minutes, or until a wooden skewer inserted into the center emerges clean. Allow the cake to cool in the pan for 10 minutes before carefully transferring it to a wire rack.
08 - While the cake is baking, begin preparing the custard sauce. In a medium saucepan, heat the whole milk over medium heat until it just begins to simmer.
09 - In a heatproof bowl, whisk together the egg yolks, granulated sugar, and cornstarch until the mixture is pale and thick.
10 - Gradually pour the hot milk into the egg yolk mixture, whisking constantly to temper the eggs and prevent scrambling. Once combined, return the entire mixture to the saucepan.
11 - Cook the custard over low heat, stirring continuously with a wooden spoon or spatula, until it thickens sufficiently to coat the back of the spoon. Do not allow the custard to boil, as this can cause it to curdle.
12 - Remove the saucepan from the heat. Stir in the vanilla extract. For an exceptionally smooth custard, strain the sauce through a fine-mesh sieve, if desired.
13 - Serve warm slices of Irish Apple Cake with generous spoonfuls of the freshly made custard sauce.