Make the custard sauce ahead of time. Bring the milk to a simmer over medium heat. Meanwhile whisk the yolks and sugar until pale. Drizzle a little of the hot milk into the egg mixture, whisking all the time. Drizzle a little more, then transfer that back into the pan of hot milk and continue cooking, stirring constantly, until the mixture coats the back of a spoon. Don't overcook or it can curdle. Stir in the vanilla. Refrigerate the custard sauce until needed.
To make the streusel topping, blend the bits of butter into the flour, sugar, and oats until the butter is incorporated and the mixture has a coarse crumbly texture. Put in the refrigerator.
To make the cake, grease a 9 inch round spring form pan.
Cream together the butter and sugar until light and fluffy. Beat in the eggs, one at a time.
Whisk together the flour, baking powder, cinnamon, and salt. Fold the dry ingredients into the butter/sugar mixture, along with the milk or cream.
Spoon the batter in to the pan, and smooth out evenly. Top with the sliced apples, and then the streusel topping.
Bake for about 50 minutes to an hour, until the top is lightly browned and a toothpick comes out without wet batter clinging to it.
Let cool a bit in the pan before removing. Dust with confectioner's sugar before serving.