Preheat the oven to 175°C/350°F (standard oven setting). Combine butter and sugar in a large bowl and mix, using a mixer fitted with the whisk attachment, until smooth and fluffy. Add the self-raising flour, salt and the egg. Fit the mixer with the dough hooks and mix until the dough comes together in a ball. Form the dough into a flat disc, wrap it in plastic wrap and place in the fridge while you make the filling and crumb topping.
Make the filling:
Peel and core the apples. Cut each apple into quarters, then slice each quarter into four even slices. In a large bowl, toss the apple slices with the sugar and cinnamon. Set aside.
Make the crumb topping:
In a medium-sized bowl, combine the cubed butter, self-raising flour, cane sugar, salt and vanilla. Rub the ingredients together with the tips of your fingers until large clumps form. Mix in the oats. Set aside.
Assemble the apple cookie crumb cake:
Get yourself a 23-cm/9-inch springform pan. Don't grease it. Remove the dough for the crust from the fridge and, using your hands, press it into the pan in an even layer. Using a fine-meshed sieve, dust the dough with the custard powder. Top with the filling, using your hands or a spoon, making sure to leave the excess apple juice in the bottom of the bowl. Sprinkle the crumb topping on top.
Bake in the middle of the oven for 50 minutes. Remove from the oven and allow to cool or serve warm. Cookie cake will keep, stored in an airtight container in the fridge, for up to three days.
Notes
(keep in mind: you only need half an egg for this recipe. To get half an egg, first beat a whole egg in a small bowl with a fork, then use half of it in this recipe).
Recipe by The Tough Cookie at https://thetoughcookie.com/2015/03/14/apple-cookie-crumb-cake/