Leaving the chocolate to set a little before topping with another biscuit means that the filling won’t spread over the edge and the biscuits won’t slide off each other.
Blitz the hazelnuts to fine crumbs, but be careful not to over-blitz – you don’t want them to become oily. Add the butter and sugar, and blitz again until really creamy, then tip the mixture into a bowl and sift in the flour. Mix with your hands, then chill in the fridge for an hour or until firm enough to roll.
Heat oven to 180C/160C fan/gas 4 and line a large baking sheet with baking parchment. Tear off teaspoon-sized chunks of mixture and roll into balls, then place on the lined sheet around 2 cm apart. Bake for 15 mins until golden brown, then transfer to a wire rack to cool.
To make the filling, heat the chocolate in a bowl in the microwave, stirring every 10 secs until fully melted. Spread a small spoonful of chocolate on half the cooled biscuits. Leave to set for 15 mins then sandwich the remaining biscuits on top. Leave the baci di dama to set completely (put them in the fridge if the kitchen is warm). Serve with coffee.