• Preheat oven to 350 degrees. Line a 9-inch springform pan with parchment paper, grease bottom and sides with oil or butter, set aside.

Streusel Topping

• Prepare the streusel topping, in a bowl add the flour, oats, sugar, and salt, stirring to combine. Using a pastry blender cut the cold butter into the flour mixture. Mix until the butter is pea sized and evenly distributed, set aside.
  ¾ cup all-purpose flour, ¼ cup oats, ½ cup granulated sugar, ⅛ teaspoon salt, 6 tablespoons unsalted butter

Irish Apple Cake

• Peel, quarter and slice apples into thin slices, about a quarter of an inch in thickness, set aside.
  3 peeled and sliced Granny Smith Apples
• In a small bowl whisk together the dry ingredients; flour, baking powder, cinnamon, and salt, set aside.
  1 ¼ cup all-purpose flour, 1 teaspoon baking powder, 1 teaspoon cinnamon, ¼ teaspoon salt
• In the bowl of a stand mixer fitted with the paddle attachment, cream the softened butter and sugar on medium speed until light, 1-2 minutes.
  8 tablespoons unsalted butter, ½ cup granulated sugar
• Add the eggs one at a time, mixing on medium speed until thoroughly combined after each addition, 1-2 minutes.  Scrape down the bowl.
  2 large eggs
• Add the dry ingredients, mix on low until just incorporated, 1 minute. Add the milk and vanilla, mix on low until just combined, turn the mixer up to medium speed, mix until thoroughly combined, 20-30 seconds. The batter will be rather thick.
  ¼ cup milk, 1 teaspoon vanilla extract
• Spread batter in an even layer in the prepared pan. Layer apple slices on top of batter, careful not to press down into the dough. Top with streusel topping. Bake for 50-55 minutes or until done. Remove cake from the oven, place on rack to cool.

Vanilla Custard Sauce

• While the cake bakes, prepare the custard sauce. In a small bowl whisk together the egg yolks, sugar, and salt.
  6 egg yolks, ⅓ cup granulated sugar, ⅛ teaspoon salt
• In a saucepan over medium heat, warm 1 ½ cups of whole milk to just steaming. Take saucepan off of heat. Temper the egg mixture by pouring a thin steady stream of the warm milk from the saucepan into the egg mixture in the small bowl. Whisk constantly.
  1 ½ cups whole milk
• Return the milk and egg mixture back to the saucepan. Heat over medium heat, gently stirring constantly until thickened and custard lightly coats the back of a spoon, 3-4 minutes.
• Whisk in the vanilla. Do not bring the mixture to a boil, doing so could cause the custard to curdle. Pour custard in a bowl to cool to room temperature, cover and store in the refrigerator until ready to serve, makes 1 ½ cups.
  1 ½ teaspoons vanilla extract
• To serve, remove cake from the springform pan, slice and serve alongside custard sauce.