Put crushed graham crackers, sugar, melted butter and salt into a bowl. Stir to combine, until all the crumbs are moistened by the butter. Add 1-2 tablespoons of the graham cracker mixture to each Mason jar. Use the back side of your tablespoon to press the crumbs down firmly into the bottom of the jar to form a crust.
Preheat oven to 350 degrees. Have two 13-by-9-inch baking pans ready.
In the bowl of a stand-mixer fitted with a paddle attachment, beat the cream cheese on medium speed until smooth and fluffy. Gradually pour in the sweetened condensed milk, followed by the eggs, zest and Key lime juice until combined.
Carefully pour the cheesecake batter into each Mason jar, filling each jar ¾ full.
Pour hot water into each of the baking pans, about one-third of the way up the sides. Gently line up the jars in the pans, 12 jars in each pan, being careful not to splash the water into the cheesecakes.
Bake the cheesecakes for 20-25 minutes, until just set. Carefully remove the pans from the oven, then gently remove the jars from the pan and set them on a wire rack to cool completely. Top with a spoonful of Key lime curd. Serve at room temperature. Store in refrigerator with lids sealed and bring back to room temperature when serving.