Preheat oven to 200 degrees C. Prepare a 26cm springform tin by moisten the base of the tin then lining with baking paper.
Sponge Cake: Using an electric mixer beat the eggs and sugar until light and fluffy.
Sift the flour, salt, and baking powder into a separate bowl then stir through the ground walnuts. Combine with the whites then pour into the prepared cake tin and smooth the top.
Half, peel and core apples. Slice down the apples hasselback style(making thin slices down from the top round edge to the flat base, but not cutting all the way through) Place the apple pieces close together on top of the sponge mixture. Place walnut halves in spaces in between them. Bake for about 25 minutes.
To Serve: Strain the jam into a small pan, mix with liqueur then melt over a low heat. Brush over cake while still hot. Dust with icing sugar.