Chop the chocolate into small chunks.
Gently heat the butter with the prosecco in a saucepan on a low heat until just melted. Take off the heat and add the chocolate.
Allow to sit for 5 minutes to melt the chocolate.
Whisk until smooth, then whisk in the raspberry powder to taste.
Cover and refrigerate until just set.
When the chocolate mixture is firm, roll into small balls and place back in the fridge until ready to serve.
Just before serving, mix together the caster sugar and freeze- dried raspberry powder.
Roll the balls in the sugar coating and serve.