Soften the gelatine sheets in a bowl with cold water, then drain them, squeezing with your hands to get out all the excess liquid. Put the cream and honey in a small saucepan on a high heat. As soon as the cream begins to froth, remove from the heat and add the gelatine sheets. Stir gently until the gelatine dissolves.
Leave the mixture to cool, then add the yoghurt and beat with a whisk until the mixture is smooth. Pour the mixture into small individual glass bowls or ramekins and leave in the fridge to cool for 3–4 hours. While the panna cottas are cooling, make the chocolate sauce.
Bring the milk to the boil, then pour over the chocolate pieces in a bowl. Stir well with a pastry spatula until the mixture is smooth, then add the butter and continue to mix until everything is well combined.
Once the panna cottas have set, serve with the chocolate sauce.