1. Preheat over to 350 degrees
2. Lets make the custard first. Brink the milk to a simmer over medium heat. While this is heating up in a small bowl whisk the yolks and sugar until combined and drizzle a little of the hot milk into the egg mixture, while continually whisking. The transfer the egg mixture into the pan of hot milk and continue cooking and stirring constantly until the mixture coats the back of a spoon. I recommend doing this slowly and using a rubber spatula to scrape the sides and bottom of pot while heating. Don't over cook or it will cuddle. Stir in vanilla and pour custard through a fine strainer to catch any lumps and place into a container, cover with plastic wrap and refrigerate until completely chilled.
3. Lets make the streusel topping. In a medium bowl (or mixer bowl) add the flour, sugar, and oats. Blend in the bits of butter until the butter is incorporated and the mixture is crumbly. Refrigerate
4. Lets make a cake! Grease a 9in round spring form pan. In a medium bowl cream the butter and sugar until fluffy and beat in the eggs one at a time. In a separate bowl whisk together flour, baking powder, cinnamon, and salt. Fold these dry ingredients into the butter/sugar mixture along with the milk/cream. Spoon this batter into the pan and smooth out evenly. Top this with the sliced apples and top with streusel topping.
5. Bake for about 50 minutes to an hour until the top is slightly browned and a toothpick comes out without wet batter sticking to it. Let cool in the pan before removing and dust with confectioner's sugar before serving and top with custard.