Put the chocolate in a medium bowl. Heat the heavy cream in a small saucepan over medium heat until just beginning to simmer. Pour the hot cream over the chocolate and allow to stand for 1 minute before stirring with a rubber spatula until smooth and combined. Place a piece of plastic wrap directly on the surface of the mixture and allow to cool to room temperature. Once cooled, add the powdered sugar and whip to soft peaks with a hand mixer.

In a second medium bowl, whisk the ricotta and amaretto until smooth and combined. Gently fold in half of the chocolate whipped cream to incorporate. When just a few streaks remain, add the remaining chocolate cream. Continue to fold until completely combined.

Spoon the mousse into a 1-gallon resealable plastic bag or piping bag. If using a resealable plastic bag,  massage it toward one corner. Snip off the corner. Insert the snipped tip of the bag into the middle of a cannoli shell and gently squeeze to fill the shell, pulling out as you go. Repeat on the opposite end of the shell. Dip each end in the chopped nuts. Repeat with the remaining shells.

Serve immediately or chill in the fridge for up to an hour. Sift some powdered sugar over the top before serving.