Preheat the oven to 325F and grease 8 ½ cup capacity metal dariole moulds (or use a muffin tin). Place the butter, chocolate, sugar, water and coffee into a heavy based saucepan and melt over a low heat, until smooth and combined. Set aside to cool slightly.
When the mixture is warm, but not hot, whisk in the egg and vanilla, then sift over the flour, baking powder and cocoa, and whisk into the mix. Divide the mixture evenly among the moulds, filling them 2/3 full. Place onto a baking sheet and bake for 20-25 minutes, or until springing back when pressed. Leave to cool in the moulds for about 5 minutes, then carefully turn out onto a wire rack to cool completely.
For the frosting:
To make the frosting, beat the butter until pale and light, then sift over the powdered sugar and beat until combined. Slowly mix in the melted, cooled chocolate, followed by the Nutella and salt. If the mixture seems too soft, place it in the fridge for a while, to firm up.
For the toffee:
To make the toffee shard, place a piece of baking paper onto a cool baking sheet and set aside. Place 2 tbsp water into the bottom of a heavy-based saucepan and add the sugar, pouring it into the middle of the pan. Place over medium heat, and let it melt, gently swirling the pan, only if the sugar is not melting evenly. Once the sugar is melted, leave it alone to boil and caramelize. This process takes about 5-7 minutes.
Keep an eye on the pan, but don’t stir it. When the sugar is a dark amber color, add in the hazelnuts, and swirl the pan to coat them in the sugar, then pour the mixture out onto the lined baking sheet, letting it spread into a thin layer. Be very careful, and don’t touch the sugar or the pan, because they will be very hot. Leave the toffee to cool and set completely, about 30 minutes.
To assemble the chocolate hazelnut mud cakes, place a big dollop of icing onto the top of each cake. Break the toffee into small shards and jam a piece into the top of each cake. Serve.