Line an 8x8-inch square baking pan with 2 sheets of parchment paper, positioning them perpendicular to each other, so that the parchment hangs about 2 inches over all 4 sides of the pan to form a sling.

Break 12 whole graham cracker sheets into rough pieces and place in a food processor. Add 2 tablespoons granulated sugar and ½ teaspoon kosher salt. Process until the crackers are the texture of sand, about 15 seconds.

Add 1 stick melted and cooled unsalted butter and pulse to combine, 6 to 8 (1-second) pulses. Transfer the mixture into the baking pan. Use the bottom of a flat measuring cup or your fingers to press the crumbs into an even layer, making sure to reach the edges and fill the corners.

Whisk ¾ cup boiling water and 1 (3-ounce) box powdered lime gelatin dessert mix together in a medium bowl, preferably metal, until the gelatin dissolves. Refrigerate to cool slightly while you prepare the rest of the filling, about 5 minutes. Do not allow the gelatin to set.

Meanwhile, wash and dry the food processor bowl and blade. Process 8 ounces room-temperature cream cheese, 1 (14-ounce) can sweetened condensed milk, ½ cup Key lime juice, and ¼ teaspoon kosher salt in the food processor until smooth and combined, about 30 seconds.

With the motor running, pour in the gelatin mixture and process until combined. Transfer over the crust and smooth out the top. Cover and refrigerate until set, at least 6 hours or up to overnight.

Grasping the excess parchment, lift the slab out of the pan and place on a cutting board. Cut into 12 bars, wiping the knife clean between each cut for the cleanest-looking slices. Top each slice with a dollop of whipped cream if desired.