For the truffles: Place 112g of the chocolate into a heat-safe bowl and either melt over a saucepan of boiling water, or in a microwave in 15 second increments. Stir until the chocolate is almost completely melted, then set aside.
Pour the cream in a small saucepan over a medium heat, whisking occasionally until it just begins to boil. Then take off the heat and pour over the chocolate, gently (don't be forceful!) stirring until combined and smooth. Then stir in the nutella and butter until smooth, then finally stir in half of the hazelnuts. Cover the mixture with cling film, pressed onto the mixture and leave at room temperature for 15 minutes. Then place in the fridge until it reaches 'scoopable' consistency, about 4 hours. Line a large baking tray with baking paper and set aside.
Once the mixture has reached 'scoopable' consistency, remove from the fridge and measure 1tsp of mixture and roll into balls. The mixture will be sticky, coating your hands in cocoa powder will help a little. Place onto the baking tray, and repeat with the rest of the mixture. Leave the truffles to 'air dry' for 15 minutes, and then re-roll each to get a neater 'ball' shape. The mixture should make 20-25 truffles.
Meanwhile melt the remaining chocolate and then dip each truffle in. When lifting the truffle out of the chocolate (a fork works best for this), tap it onto the side of the bowl to remove the excess chocolate. Then place back onto the baking tray. Sprinkle the truffles with the remaining hazelnuts and leave aside until the chocolate has completely set.
For the cake: Preheat the oven to 170C/Gas Mark 5 and line three 18cm-round cake tins and set aside.
In a small bowl mash the bananas together, making sure to leave some chunky pieces and set aside. Then roughly chop the hazelnuts and in a dry frying pan, roast them until they are just toasted. Set these aside too.
In a large bowl, using an electric whisk, beat the oil and the sugar together. Then add the eggs in one at a time, making sure they are fully incorporated. Beat the mixture for 5 minutes until the mix is thick and creamy. Mix in the banana, hazelnuts, flour, bicarbonate of soda, baking powder, cinnamon and salt until just combined and the batter is smooth. Be careful not to overmix this.
Evenly spoon the mixture into your prepared baking tins and bake for 20-25 minutes or until a skewer inserted comes out almost clean (It's okay to leave it a little undercooked as the cake will continue to cook in the tins). Leave the cakes in the tins for 10 minutes, before placing them on wire racks to cool completely.
For the salted caramel sauce: Add the sugar and golden syrup into a small saucepan with a few splashes of water. Let it simmer over a medium heat for 3 minutes, stirring continuously until you have a smooth caramel. Then add the butter, cream and sea salt and keep stirring. Be careful as the mixture will bubble up. Turn the heat down, and cook for another minute until the mixture is thickened. Then pour into a heat-safe bowl, let cool completely and set aside.
For the frosting: Melt the butter in a small saucepan over a low temperature, and keep cooking until it turns a nutty golden brown colour. Keep an eye on it, as it can go black quickly and you don't want that! Take it off the heat.
In a large bowl, add the icing sugar and whilst using an electric whisk slowly pour in the melted butter. The mixture will be very thick and lumpy. Add in the milk 1tbsp at a time, whisking until you reach a smooth buttercream consistency. Then set aside.
To assemble: Place one of the cakes onto a serving plate or cake board, and spread on a 1/4 of the frosting. Then place another cake on top and spread on another 1/4 of frosting. Finally add the final cake and cover the top and sides with the remaining frosting. Using a palette knife, scrape around the sides so the frosting is flush to the cake.
Then using a spoon drizzle the salted caramel sauce around the edges of the cake, coaxing it off the edge to create 'drips' of different sizes. Then fill in the top of the cake with the remaining sauce. Finally add truffles onto the top of the cake.