Melt 300g of milk chocolate in a bowl over a pan of simmering water without letting the bowl touch the water.
Heat the chocolate to 42°C, then remove from heat and cool to 31°C to temper.
Coat each fluted cup in the mould quickly using a brush or teaspoon.
Scrape excess chocolate from the top of the mould and save for later use.
Refrigerate the moulds for 20 minutes until the chocolate sets.
Mix coffee essence with enough water to make 50ml, add sugar, and boil in a pan for one minute.
Stir the coffee liquid into the reserved 50g of chocolate to melt it and chill in the fridge until thick.
Spoon the thickened coffee mixture into the moulds and leave to set overnight.
Temper the remaining milk chocolate to seal the whips.
Spoon the tempered chocolate over the filling to cover completely.
Place a walnut on each whip and chill for 30 minutes before releasing from the mould.
Serve the chocolate whips with coffee.