Preheat the oven to 350F. Spray 9 inch springform pan with oil. Put parchment on bottom and a strip on sides.

Cake: Cream butter and sugar until light and fluffy. Beat in the eggs, one at a time. Beat in milk and vanilla.

Whisk flour, baking powder, cinnamon, and salt. Fold into butter/sugar mixture. Fold in raisins, if using.

Spoon batter into prepared pan, and smooth out. Top with apples in somewhat even layer, and then the streusel topping.

Bake for 50 minutes to an hour, until the top is lightly browned and a toothpick comes out clean.

Cool slightly in pan before removing. Run knife around sides. Dust with confectioners sugar before serving.

Custard: Make while cake is baking. Bring milk to a simmer over medium heat. Whisk yolks and sugar until well combined. Drizzle a little of the hot milk into the egg mixture, whisking. Drizzle more, then transfer into the pan of hot milk. Continue cooking, stirring constantly, until the mixture coats the back of a spoon. Use a silicone spoon or spatula to scrape the bottom and sides of the pan as it heats. The mixture will be thickend and will continue to thicken as it cools. Dont overcook or it can curdle. Stir in vanilla.

Pour the custard into a heat proof jar or bowl and place a piece of plastic wrap onto the surface to avoid a skin as it cools. Refrigerate until completely chilled.

In a medium-sized bowl, whisk flour, brown sugar, granulated sugar, and salt. Drizzle butter over the mixture and use a fork to toss the ingredients together until flour absorbed but still clumpy. Dont over-mix or it will become a paste. Refrigerate.

Scatter evenly over muffins, pie, or coffee cake before baking. Adding streusel might add a minute or so to baking time.