Preheat the oven to 190℃ /170℃ fan/gas mark 5. Grease and line the base of a 20cm round cake tin.
Cream the butter and sugar together until light and fluffy, then beat in the egg yolks one at a time and beat well after each addition.
Add the flour mix until just combined. Dissolve the coffee granules in the milk and add to the cake mixture along with the chopped hazelnuts.
Whisk the egg whites until standing in stiff peaks, then carefully fold into the mixture with a spatula or metal spoon, taking care not to knock out all of the air.
Spoon into the prepared tin and level. Bake for 25–30 minutes until firm but springy to the touch. Allow to cool in the tin for a few minutes before transferring to wire rack to cool completely.
Meanwhile, place the apples in a pan with 2tbsp of the apricot jam, the lemon zest and juice. Cover and cook for about 5 minutes until tender. Leave to cool.
When the cake is completely cold, split into two layers and sandwich back together with the apples and a little more apricot jam if desired. Dust the top with icing sugar and serve.
Notes
We added coffee to the cakes but I think it would taste just as good without so have made this optional. Freeze: Freeze for up to 1 month