Let’s bake this delightful Irish Apple Cake step-by-step:

1. Make the Custard Sauce (Make Ahead Recommended):

Bring the milk (or cream/half-and-half) just to a simmer in a medium saucepan over medium heat. Do not boil.

Meanwhile, in a medium heatproof bowl, whisk the egg yolks and sugar together until well combined and slightly lightened in color.

Temper the yolks: Slowly drizzle about half a cup of the hot milk into the egg yolk mixture while whisking constantly. This prevents the yolks from scrambling.

Pour the warmed yolk mixture back into the saucepan with the remaining hot milk.

Cook over medium-low heat, stirring constantly with a silicone spoon or spatula (scraping bottom and sides), until the mixture thickens enough to coat the back of the spoon (an instant-read thermometer should register 170-175°F / 77-79°C). Do this slowly and gently; do not boil or it will curdle.

Immediately remove the saucepan from the heat. Stir in the vanilla extract.

Pour the custard sauce through a fine-mesh sieve into a heatproof jar or bowl. This catches any potential small lumps.

Press a piece of plastic wrap directly onto the surface of the custard to prevent a skin from forming as it cools.

Refrigerate until completely chilled (at least 2 hours, or preferably overnight).  

2. Make the Streusel Topping:

In a medium bowl, combine the all-purpose flour, granulated sugar, and rolled oats.

Add the cold, small pieces of butter.

Using a pastry blender, two knives, or your fingertips, cut the butter into the dry ingredients until the butter is incorporated and the mixture has a coarse, crumbly texture with some larger pea-sized bits remaining.

Place the streusel mixture in the refrigerator to keep cold while you make the cake batter.

3. Prepare Oven and Pan:

Preheat the oven to 350°F (175°C). Grease a 9-inch springform pan (or regular cake pan). Lining the bottom with parchment is recommended.

4. Make the Cake Batter:

In a large bowl (using an electric mixer), cream together the room temperature butter and granulated sugar until light and fluffy.

Beat in the room temperature eggs, one at a time, mixing well after each addition.

Whisk together the all-purpose flour, baking powder, cinnamon, and salt in a separate medium bowl.

With the mixer on low speed, add the dry ingredients to the butter/egg mixture alternately with the milk or cream (start and end with dry). Mix just until combined after each addition. Do not overmix.

5. Assemble the Cake:

Spoon the prepared cake batter into your prepared springform pan. Smooth the top out evenly with an offset spatula or the back of a spoon.

Arrange the thinly sliced apples over the top of the batter in an even layer. It’s okay if they overlap slightly; no need for perfect arrangement.

Sprinkle the chilled streusel topping evenly over the apples.

6. Bake the Cake:

Bake for about 50 minutes to 1 hour, until the streusel topping is lightly browned and a toothpick inserted into the cake portion (not just apples) comes out clean or with a few moist crumbs clinging to it.

7. Cool the Cake:

Let the cake cool a bit in the pan on a wire rack (about 15-20 minutes).

If using a springform pan, carefully release and remove the sides. If using a regular pan, let cool longer before attempting to invert (or serve from the pan). Let cool further on the rack.

8. Serve:

Dust the warm or room temperature cake generously with confectioner’s sugar just before serving.

Serve slices warm, accompanied by the chilled homemade custard sauce (Crème Anglaise). Enjoy!