Cream butter and sugar in a large bowl (stand or hand mixer), until smooth. There should be minimal lumps of butter
Add egg and vanilla and mix for 30 seconds until the egg is just combined.
Add flour, small bits at a time, making sure to scrape down the sides with a spatula to incorporate all ingredients.
Turn the dough onto a piece of plastic wrap/cling wrap, and shape into a ball. Cover and refrigerate for 30 minutes.
Preheat oven to 190C/350F
Grease a 23cm/9inch round cake tin
Divide dough into equal halves.
Flour your hands.
Press half of the dough into your greased pan as evenly as possible. Use your hands to work the dough to the edges.
Top with the apple pie filling.
Roll the other half of the dough into a rough round that fits the size of the pan. Place the dough on top of the pie filling and use your hands to gently press the dough into any missing gaps. It doesn't matter if it's not perfect because you can flip the cake when it's cooked and no one will see the bottom!
Bake for 25-30 minutes in a fan forced ovenm or 35-40 minutes in non-fan forced, or until top is browned and cake it cooked through.
Cool for 20 minutes in the tin, then turn out onto a wire rack and cool completely.
Transfer to a plate and serve with a dusting of icing sugar and some cream and/or ice cream.