Whisk the instant pudding mix with the sweetened condensed milk in a large mixing bowl until completely smooth with no powder lumps. Add the fresh lime juice and zest, whisking until well combined.

In a separate chilled bowl, whip the heavy cream and vanilla extract until soft peaks form (about 2-3 minutes with an electric mixer). The cream should fall in gentle ribbons when you lift the beaters.

Gently fold the whipped cream into the lime mixture using a rubber spatula, being careful not to deflate the air. Use a folding motion from bottom to top, rotating the bowl as you go.

Fold in the mini marshmallows gently until distributed throughout the mixture.

Sprinkle in most of the crushed graham crackers, reserving about 1/4 cup for garnish. Fold them in gently. If using pecans, add them now.

Transfer the mixture to a large serving bowl or individual dessert cups. Cover tightly with plastic wrap, pressing it directly onto the surface to prevent a skin from forming.

Refrigerate for at least 2 hours, though overnight is even better for the flavors to meld and texture to set properly.

Just before serving, top with dollops of fresh whipped cream and sprinkle with the reserved graham cracker crumbs and additional lime zest.