Sunken chocolate, apple and hazelnut cake

1. Preheat the oven to 160°C. Line the base of a 20cm springform tin with baking paper and grease the tin with oil spray.  
2. Put the hazelnuts on a tray and roast for 10-15 minutes until they start to golden. Remove from the oven and place in a clean tea towel. Leave to steam for 5 minutes, then rub nuts while in the tea towel to remove the skin. Allow to cool. Pulse half the hazelnuts in a food processor to a fine crumb and mix in the flour. 
3. Melt the chocolate in a heatproof bowl set over a pan of simmering water (or do in short bursts in the microwave). Whisk in the olive oil, then add 100g caster sugar and stir well. Remove from the heat and cool for 5 minutes, then add the ground hazelnuts, flour mix and egg yolks. 
4. Halve the apples, core and thinly slice and drizzle with the lemon juice. Set aside. 
5. Whisk the egg whites to soft peaks with an electric beater and gradually add the remaining sugar until the mix is thick and glossy. Fold a little of the egg whites into the chocolate to loosen, then gently fold through the rest of the whites into the chocolate.  
6. Put mix into the tin, arrange the apple slices on top and scatter with the remaining hazelnuts. Bake for 60-70 minutes or until a skewer inserted comes out clean. Cool in the tin (it will start to sink and crack a little).  

Top Tips:

Flour makes the cake a little denser, but this can be omitted if you prefer a completely gluten-free cake.   

For something a little less fussy swap hazelnuts for ground almonds (omit the roasting stage) and scatter the top with sliced almonds.