Let’s create this heartwarming Irish Apple Cake, starting with the essential custard sauce, which is best made ahead to chill completely.

Make the Custard Sauce (Crème Anglaise): Prepare an ice bath (a large bowl with ice and cold water) and have a clean heatproof jar or bowl ready near the stove. Separate the 6 eggs, placing the yolks in a medium bowl (reserve whites for another use). Add the 6 Tbsp granulated sugar to the yolks and whisk together vigorously until pale and slightly thickened.

In a medium saucepan, heat the 1 1/2 cups of milk (or half-and-half/cream) over medium heat until it just reaches a simmer (small bubbles around the edges, steam rising – do not boil). If using a vanilla bean, add the pod and scraped seeds to the milk while heating, then remove pod before tempering.

Temper the eggs: While whisking the yolk/sugar mixture constantly, very slowly drizzle in about half a cup of the hot milk. This gradually raises the temperature of the yolks without cooking them. Continue whisking and slowly pour in a little more hot milk. Pour the now-warmed yolk mixture back into the saucepan containing the rest of the hot milk.

Return the saucepan to medium-low heat. Cook, stirring constantly and scraping the bottom and sides of the pan with a heatproof spatula or wooden spoon, until the mixture thickens enough to coat the back of the spoon (nappe stage). To test, dip the spoon, lift it, and run your finger through the custard on the back; if the line holds clearly, it’s ready. This takes patience and gentle heat (5-10 minutes). Do not let it simmer or boil, or the eggs will scramble.

Immediately remove the custard from the heat. Stir in the 1 1/2 tsp vanilla extract (if using extract). Pour the finished custard through a fine-mesh sieve directly into your clean jar or bowl – this catches any potential lumps for a perfectly smooth sauce. Press a piece of plastic wrap directly onto the surface of the custard to prevent a skin from forming. Place the custard in the refrigerator to chill completely (at least 2-3 hours, or ideally overnight).

Make the Streusel Topping: In a medium bowl, combine the 3/4 cup all-purpose flour, 1/4 cup rolled oats, and 1/2 cup granulated sugar. Add the 6 Tbsp of cold, cubed butter. Using your fingertips, a pastry blender, or pulsing briefly in a food processor, cut the butter into the dry ingredients until the butter is incorporated and the mixture resembles coarse, crumbly meal with some larger pebble-sized bits. Do not overwork into a paste. Place the streusel mixture in the refrigerator to keep the butter cold while you make the cake batter.

Make the Apple Cake: Preheat the oven to 350°F (175°C). Grease a 9-inch springform pan well (buttering is nice). Prepare the apples: peel, core, and thinly slice the Granny Smith apples (about 1/8-1/4 inch thick). Set aside.

In a large bowl (or stand mixer bowl with paddle), cream the room temperature butter and the 1/2 cup granulated sugar together on medium-high speed until light and fluffy (2-3 minutes). Beat in the room temperature eggs one at a time, mixing well after each addition. Beat in the 3 Tbsp room temperature milk or cream.

In a separate medium bowl, whisk together the 1 1/4 cups all-purpose flour, 1 tsp baking powder, 1 tsp cinnamon, and 1/8 tsp salt. Add these combined dry ingredients to the creamed butter/egg mixture. Fold gently with a spatula (or mix on lowest speed) just until combined. Do not overmix. The batter will be relatively thick.

Spoon the cake batter into your prepared 9-inch springform pan. Use an offset spatula or the back of a spoon to spread the batter evenly across the bottom. Arrange the thinly sliced apples over the batter in an even layer (no need for perfect concentric circles, just cover the batter). Sprinkle the chilled streusel topping evenly all over the apples.

Bake for approximately 50 minutes to 1 hour. The top should be golden brown, the streusel crisp, and a wooden toothpick or skewer inserted into the center (through the apples and into the cake) should come out clean or with moist crumbs, but no wet batter.

Let the cake cool in the springform pan on a wire rack for at least 15-20 minutes before attempting to release the sides of the pan. Let it cool further on the rack before serving.

To serve, dust the cooled (or slightly warm) cake generously with confectioner’s sugar. Slice into wedges and serve with a generous drizzle or pool of the chilled Crème Anglaise (custard sauce) alongside each slice.