Cake:

Preheat the oven to 350°F (175°C) and prep an 8-inch round cake pan by spraying with pan spray.
In a large bowl, use the paddle attachment to cream together the butter and sugar until light and fluffy, about 2 minutes at medium-high speed. Add the vanilla, milk, and eggs and mix until fully combined. Scrape down the sides.
In a medium bowl sift together the flour, baking powder, cinnamon, and salt. Add the dry ingredients to the wet and mix until just barely fully combined.

Add the chopped apples to the batter and fold in by hand until evenly distributed throughout. This is an apple-heavy recipe, so should be about 50/50 apples to the batter. Pour the batter into the prepared cake pan and spread until smooth.
Top with the streusel topping and bake for 45-50 minutes, until the streusel is lightly golden brown and a toothpick comes out clean from the center of the cake. Allow to cool at least 15 minutes before serving with warm custard sauce.

Streusel:

In a medium bowl, combine the flour, oats, sugar, and salt. Cut the butter into small cubes and toss with the dry ingredients.
Using your fingers, rub the butter into the mixture. It’s best to squish the pieces of butter with your thumb and forefingers to flatten and then continue to rub it all together until you get the texture of coarse breadcrumbs.
Keep the streusel chilled in the fridge until your cake batter is ready to be topped.

Custard Sauce:

Add the milk and salt to a medium saucepan. Heat over medium heat until steaming. Don’t let it boil! It should be just barely to the point of simmering.
In a medium bowl, whisk together the egg yolks and sugar until pale and thick. This will take 2-3 minutes by hand, or about 30 seconds if you use a hand mixer.

Remove the milk from the burner and slowly whisk it into the egg yolk mixture, slowly bringing the egg yolks up in temperature. Once all of the milk has been whisked into the eggs, pour the mixture back into the saucepan and heat over medium heat while constantly whisking until thickened enough to coat the back of a spoon, about 3-4 minutes.
Remove from the heat and stir in the vanilla. Serve warm over the cake.