In a large mixing bowl, combine the heavy cream, whole milk, sugar, salt, sweetened condensed milk, key lime juice, and lime zest.
Whisk until the sugar is fully dissolved and the mixture is smooth and creamy.
Add a small spoonful of graham cracker crumble to the bottom of each popsicle mold.
Pour the key lime mixture into the molds, leaving a little room at the top for expansion.
Add another light layer of graham cracker crumble near the top of each popsicle and gently swirl with a popsicle stick or knife if desired.
Insert the popsicle sticks and freeze for 5–6 hours, or until completely solid.
To release the popsicles, run the molds under warm water for a few seconds, then gently pull them out.
Garnish with extra lime zest and additional graham cracker crumble before serving if desired.