Finely chop the semisweet and bittersweet chocolate and place in a heatproof bowl.

Heat heavy cream in a saucepan over medium heat until it just begins to boil.

Pour the hot cream over the chopped chocolate.

Whisk the mixture until the chocolate is completely melted and smooth.

Stir in the hazelnut liqueur and vanilla extract.

Pour the chocolate mixture into a pie dish or shallow container.

Refrigerate for approximately 45 minutes, or until the mixture is firm enough to handle.

Line a baking sheet with parchment paper.

Using two spoons, scoop small, rounded teaspoons of the chocolate mixture.

Drop each spoonful onto the prepared parchment-lined baking sheet.

Rinse your hands under cold water and dry thoroughly.

Roll each chocolate portion into a smooth, round ball using your hands.

Roll the chocolate balls in cocoa powder until fully coated.

Place the coated truffles on a serving dish.

If your hands become too warm, rinse them under cold water and dry before continuing to roll.

Refrigerate the truffles until ready to serve to allow them to fully set.