For the Cake:

1. Preheat the oven to 350°F and grease an 8-inch round cake pan with cooking spray.
2. In a large bowl, cream together the butter and sugar with the paddle attachment until light and fluffy (about 2 minutes on medium-high speed). Add vanilla, milk, and eggs, mixing until fully combined.
3. In a medium bowl, sift together the flour, baking powder, cinnamon, and salt. Add the dry ingredients to the wet ingredients and mix until just combined.
4. Gently fold in the diced apples until evenly distributed throughout the batter. The mixture should have a 50/50 ratio of apples to batter.
5. Pour the batter into the prepared cake pan and spread evenly.
6. Top with the prepared streusel topping and bake for 45-50 minutes, until the streusel is golden brown and a toothpick inserted into the center comes out clean. Let the cake cool for at least 15 minutes before serving with the custard sauce.

For the Streusel Topping:

1. In a medium bowl, mix together the flour, oats, sugar, and salt. Cut the butter into small cubes and toss with the dry ingredients.
2. Using your fingers, rub the butter into the mixture until it resembles coarse breadcrumbs.
3. Keep the streusel chilled in the fridge until you’re ready to top the cake.

For the Custard Sauce:

1. In a medium saucepan, heat the milk and salt over medium heat until it begins to steam, being careful not to boil.
2. In a medium bowl, whisk together the egg yolks and sugar until pale and thick (2-3 minutes by hand, or 30 seconds with a hand mixer).
3. Slowly whisk the steaming milk into the egg mixture, gradually warming the eggs.
4. Pour the mixture back into the saucepan and cook over medium heat, whisking constantly, until it thickens enough to coat the back of a spoon (3-4 minutes).
5. Remove from heat and stir in the vanilla extract. Serve warm over the cake.