In a large bowl, whisk together condensed milk, Key lime juice, zest, mint, and salt until well combined.
In a separate large bowl, beat 2 cups cream and ½ teaspoon vanilla with a mixer at medium-high speed until stiff peaks form. Fold cream mixture into condensed milk mixture in three additions until well combined.
Place about one-third of graham crackers in an even layer over the bottom of a 13x9-inch casserole dish. Spread half of cream mixture (about 3½ cups) over crackers; cover with another third of graham crackers and remaining cream mixture. Top with remaining third of graham crackers.
In a large bowl, beat honey, remaining 1½ cups cream, and remaining 1 teaspoon vanilla with a mixer at medium-high speed until stiff peaks form. Spread whipped cream over graham crackers.
Loosely cover and refrigerate for at least 3 hours or up to overnight before serving. Garnish with zest, if desired.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/key-lime-icebox-cake/