In a small bowl, mix together the graham cracker crumbs, sugar, and salt. Pour in the melted butter and mix with a fork until the texture resembles wet sand. Set aside.
In a medium bowl, beat the cream cheese, condensed milk, and 1 tablespoon heavy cream with an electric hand mixer until smooth and thick and the beaters leave a ribbon when lifted out of the bowl, 3-4 minutes. Using a flexible spatula, fold in the key lime juice and ½ teaspoon key lime zest until just combined.
Place the jars on a baking sheet. Spoon 2 scant tablespoons of the crumb mixture into each jar. Reserve the remaining crumbs. Press down firmly on the crumbs in the jars to form a packed, even layer.
Divide the filling evenly between the jars, about ¼ cup (25 G) per jar. Cover the jars with the lids and refrigerate for at least 1 hour, or up to overnight.
In a medium bowl, whip the remaining ⅓ cup (80 ML) heavy cream and the powdered sugar with an electric hand mixer until soft peaks form.
When ready to serve, top each jar with a dollop of whipped cream, the remaining cookie crumbs, and the remaining ½ teaspoon lime zest. Serve immediately.
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