01 - Preheat your oven to 350°F. Thoroughly grease and line a 9-inch round cake pan, ensuring non-stick surfaces are ready for baking.
02 - In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg until uniformly combined.
03 - Add the cold, cubed unsalted butter to the dry ingredients. Using your fingertips, rub the butter into the flour mixture until the consistency resembles coarse crumbs.
04 - Stir in the granulated sugar. Subsequently, fold in the thinly sliced Granny Smith apples, ensuring each slice is well coated within the mixture.
05 - In a separate bowl, whisk together the large eggs, whole milk, and vanilla extract until thoroughly blended.
06 - Gently pour the wet ingredient mixture into the dry ingredients and apple mixture. Mix with a spatula or wooden spoon until just combined; avoid overmixing to maintain a tender crumb.
07 - Transfer the prepared batter into the 9-inch round cake pan, spreading the top evenly. Bake for 45 to 50 minutes, or until a wooden skewer inserted into the cake's center emerges clean. Allow the cake to cool in the pan for 10 minutes before carefully inverting it onto a wire rack.
08 - While the cake bakes, begin preparing the custard sauce. In a medium saucepan, heat 2 cups of whole milk over medium heat until it reaches a gentle simmer, just before boiling.
09 - In a separate heatproof bowl, vigorously whisk the 4 large egg yolks, 1/2 cup granulated sugar, and 1 tablespoon cornstarch until the mixture becomes pale and thick.
10 - Gradually pour a small ladleful of the hot milk into the egg yolk mixture while whisking constantly. This tempering step prevents the eggs from scrambling. Once tempered, slowly incorporate the entire yolk mixture back into the saucepan with the remaining hot milk, whisking continuously.
11 - Continue cooking the custard over low heat, stirring constantly with a wooden spoon or heatproof spatula. Cook until the sauce visibly thickens and coats the back of your spoon. Crucially, do not allow the custard to boil, as this can cause it to curdle.
12 - Remove the saucepan from the heat. Stir in 1 teaspoon of vanilla extract. For an exceptionally smooth custard, strain it immediately through a fine-mesh sieve into a clean bowl.
13 - Present warm slices of the Irish Apple Cake, generously adorned with spoonfuls of the creamy homemade custard sauce.