I have used dark chocolate, but the finished product has a milk chocolate flavour, because of all the cream in the recipe. You can use milk chocolate, but I feel that the chocolate flavour then tends to be overly subtle in the finished product.
Take the double single cream out of the fridge to warm up. Make sure your ice-cream maker is ready to use, and have all your equipment handy (you don't want to have to break off in the middle of making this to rummage in a drawer)
Melt chocolate by the usual method in a pyrex bowl over simmering water Cheap Cigarettes Outlet, or in a double boiler. When it has melted, add the double cream to the chocolate in the warm bowl, and stir well. Then add the single cream, continuously gently stirring the mixture until it is perfectly even.