Make the Cake
Cut the butter into the flour until the mixture resembles coarse crumbs.
Add the milk and mix into a smooth dough.
Wrap and refrigerate for 1 hour.
Divide the dough into 4–6 equal portions.
Roll each portion into a thin round about 3 mm thick.
Bake each layer in a preheated oven at 190°C (375°F) until lightly golden.
Allow the layers to cool completely.
Prepare the Apple Filling (Optional)
Cook the diced apples with the sugar and water until soft and thickened.
Cool completely
Make the Vanilla Cream
Whisk the eggs with sugar.
Stir in the flour and vanilla.
Heat the milk until almost boiling.
Slowly whisk the hot milk into the egg mixture.
Return to low heat, stirring constantly until thickened.
Cool completely.
Beat the butter until fluffy.
Gradually beat the cooled custard into the butter.
Stir in brandy if using.
Assemble
Spread vanilla cream between each cake layer.
Add the apple filling to one layer if desired.
Stack the layers evenly.
Cover the top with cream.
Dust generously with powdered sugar.
Chill for several hours before slicing.