Step 1 In a medium, heavy-bottom saucepan, bring 2 cups of cream to a boil with the cardamom seeds. Remove from heat, cover the top of the pan with plastic wrap and allow to steep for 10 minutes. Remove the plastic and bring the cream to a boil again. Remove from heat and strain the seeds.
Step 2 Place the milk chocolate in a medium bowl. Pour the hot cream over the chocolate and stir gently to combine, melting the chocolate and incorporating with the cream. Cool the mixture to room temperature (it is important that the chocolate be at room temperature before continuing with the recipe; the chocolate can be neither too hot nor too cold).
Step 3When the melted chocolate has come to room temperature, whip the remaining 2 cups of heavy cream to soft peaks (be careful not to over-beat). Gently fold one-third of the whipped cream into the chocolate mixture, then fold in the remaining two-thirds to form a mousse. The mousse will be very loose at this point; it will thicken as the assembled dessert chills.
Step 4Gently fold the chopped bittersweet chocolate into the mousse and set aside.
Step 5Assemble the dessert: Place one-third of the mousse in the bottom of a trifle dish or decorative bowl. Top the mousse with a layer of sliced pound cake (4 to 5 slices). Add one-third each of the raspberries, blueberries and sliced banana.
Step 6Repeat with a second layer of mousse, topped with the remaining pound cake and another third of the fruit. Top with the final third of the mousse, decorating the top of the mousse with the remaining fruit.
Step 7Place the mousse, uncovered, in the refrigerator for 1 to 2 hours to thicken and chill. To serve, scoop the mousse into bowls.