Preheat the oven to 180°C, gas mark 4. Grease and line 2 tall 22cm diameter cake tins. Line a large baking tray with baking paper.
Put 450g butter and 450g caster sugar in a bowl and beat with an electric hand whisk until light and fluffy, then beat in the eggs, one at a time.
Add half the coffee granules to a small bowl with 1½ tbsp boiling water. Stir until dissolved, then pour into the batter. Sift in the flour and fold together until well combined.
Divide the batter between the tins and bake for 40-45 minutes, or until a skewer inserted into the centre comes out clean. Leave to cool in the tins for 10 minutes, then turn out onto a wire rack to cool completely.
Set a non-stick frying pan over a medium-high heat. Add the hazelnuts, cinnamon, remaining caster sugar and 15g butter. Stir constantly for 4-5 minutes, until the butter and sugar have melted and the nuts are coated in a golden caramel. Tip onto the prepared baking tray and use a fork to separate the nuts. Leave to set for 10 minutes.
Once the cakes have cooled, slice each one in half horizontally to give you 4 discs of sponge.
For the filling, put the remaining butter in a bowl and beat in a third of the icing sugar. Gradually add the rest of the icing sugar, beating until well combined.