Step 1

Preheat oven to 350° F. Line a 9-inch springform pan with parchment paper round and grease with nonstick spray. Set aside.

Step 2

Whisk together the flour, oats, sugar and salt. Add butter and use a pastry cutter (or two forks) to cut it into the dry ingredients until a sand-like mixture forms. Keep streusel in the refrigerator while preparing the cake batter.

Step 3

In a large bowl, whisk together flour, baking powder, cinnamon and salt. Set aside.

Step 4

In a separate large bowl, beat butter and sugar until creamy, about 1 minute. Beat in eggs one at a time, then mix in vanilla and milk. Scrape down the sides and bottom of the bowl as necessary. Add flour mixture and mix just until combined. The batter will be thick.

Step 5

Transfer batter to prepared pan. Arrange apple slices on top in one layer. Sprinkle streusel on top.

Step 6

Bake at 350° F for 55 to 60 minutes. Let the cake cool in the pan for about 15 minutes before running a knife along the edge and removing the sides of the springform pan. Cool completely.

Step 7

In a medium bowl, whisk together egg yolks, sugar and vanilla until pale, about 2 minutes. Set aside.

Step 8

In a medium saucepan, heat milk until scalding hot (the edges of the pan will just start to simmer).

Step 9

Temper the egg yolks. Slowly pour about ½ cup of the hot milk into the bowl of the egg mixture, whisking constantly. Pour egg mixture back into the saucepan, again whisking constantly.

Step 10

Continue cooking the custard over medium heat until the custard thickens, about 4 minutes. Remember to stir constantly so the mixture doesn’t burn. Custard is done when it coats the back of a spoon. Transfer to a sauce boat or serving bowl.

Step 11

Slice the cake and serve with a drizzle of custard sauce.