Preparation

• Preheat oven to 350F and grease a 9-inch round spring form pan.
• Put the streusel ingredients in a small bowl and blend everything together with your fingertips until it has a nice even crumbly texture and there is no more raw flour. Make sure to break down any large clumps of butter. Refrigerate.
• Toss the apple slices with a little lemon juice to prevent browning.
• Cream together the butter and sugar until light and fluffy. Beat in the eggs, one at a time.
• Whisk together the flour, baking powder, cardamom, 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 crumble topping.
• Bake for about 50 minutes, until the top is lightly browned and a toothpick comes out without wet batter clinging to it. Lay a loose layer of foil over the top if the crumble is browning too quickly.
• Let cool a bit in the pan before removing the sides. Dust with confectioner's sugar before serving.
• To make the custard sauce 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. Add a little more, while whisking, then transfer that back into the pan 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. I like to run the sauce through a strainer to catch any bits of cooked egg or lumps.
• Refrigerate the custard sauce until needed.