Place chocolate in a microwave-safe bowl and heat on high for 20 seconds. Stir well, and continue warming in 20-second intervals until completely melted.
Line a baking sheet with a piece of parchment paper. Working one at a time, dip the tops of mini cream puffs into the melted chocolate, place on the prepared baking sheet and sprinkle with sanding sugar. Repeat with all 3 packages of the mini cream puffs.
On a small plate, arrange both the mini cream puffs and the chocolate hazelnut cream puffs (in an alternating pattern) in an 8-inch wide circle, applying a smear of buttercream to the bottom of each one to hold it in place. Fill in the centre of the circle with cream puffs as well, using the same method.
Repeat with the remaining cream puffs and frosting, making each tier slightly smaller than the one below it, creating a tree shape. Garnish with fresh mint leaves, tucking them in between the cream puffs. Dust the entire tree with a generous sprinkling of powdered sugar.