Step 1: Prepare the pudding/custard. If using instant pudding mix, whisk the pudding mix with 4 cups of cold milk in a large bowl for 2 minutes until thickened. For a firmer set, use slightly less milk (e.g., 3.5 cups). Let it set in the refrigerator for 5-10 minutes.
Step 2: Prepare the whipped cream. In a separate large bowl, using an electric mixer, whip the heavy cream, powdered sugar, and vanilla extract on high speed until stiff peaks form.
Step 3: Assemble the first cake layer. Arrange half of the cubed cake pieces in the bottom of a large (3-4 quart) clear glass trifle bowl. If desired, lightly drizzle the cake with half of the raspberry liqueur or sherry.
Step 4: Add the first fruit layer. Scatter half of the fresh raspberries evenly over the cake layer.
Step 5: Apply the first custard layer. Carefully spoon half of the prepared vanilla custard over the raspberries, spreading it gently to cover the fruit and cake.
Step 6: Add the first whipped cream layer. Gently spread half of the whipped cream over the custard layer.
Step 7: Repeat the layers. Repeat with the remaining cake (drizzling with liqueur if using), remaining raspberries, remaining custard, and finally top with the remaining whipped cream.
Step 8: Chill and garnish. Cover the trifle lightly with plastic wrap and refrigerate for at least 2-4 hours to allow the flavors to meld and the cake to absorb moisture. Before serving, garnish with a few extra fresh raspberries and fresh mint leaves.