Any basic chocolate truffle recipe can be modified to create Dark Chocolate Truffles, Milk Chocolate Truffles or White Chocolate Truffles by following a few of these rules.
In the following recipe you can make the substitutions as indicated when you want to change the flavor of your truffles. That being said, the texture (hard or soft) of every recipe for chocolate truffles is dependent on the amount of cocoa solids in the chocolate you are using. A chocolate with less cocoa solids such as a 35% dark chocolate will produce a softer chocolate truffle than one that is 70%. You'll have to experiment a little to find the texture you want.
Also the proportion of chocolate to cream determines the stiffness of the resulting product. Add more cream and you will get a softer product. If you need to stiffen the mixture - add more chocolate.