In a saucepan, bring the cream to a boil. Immediately remove from heat, add the sprigs of mint, cover and let steep 30 minutes.
Remove the mint and measure the cream. Add additional cream if necessary to bring it up to 150ml.
Finely dice the chocolate and place in a bowl. Bring the cream to a simmer and pour over the chocolate. Let stand 1 minute then beat with a small whisk until smooth and silky. Cover with plastic wrap and chill in the fridge 1 hour.
Shape into 2cm balls, rolling between your hands. Roll in cocoa powder and serve on a plate.