Step 1

Thaw 1 (8-ounce) container frozen whipped topping if needed. Juice 1 medium lemon into a large bowl until you have 2 tablespoons. Add 1 cup boiling water and 1 (3-ounce) box powdered lemon gelatin dessert mix, and whisk until dissolved. Add 3/4 cup cold water and whisk until combined. Cover and refrigerate until thickened but not set, 1 hour to 1 hour and 15 minutes.

Step 2

Line an 8x8-inch 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.

Step 3

Melt 1 stick unsalted butter in a small microwave-safe bowl in 20-second bursts until mostly melted, 40 to 60 seconds total (it will melt completely as it sits). (Alternatively, melt the butter in a small saucepan over medium heat.) Set aside to cool slightly.

Step 4

Break 12 whole graham cracker sheets into rough pieces and place in a food processor fitted with a blade attachment. Add 2 tablespoons granulated sugar and 1/2 teaspoon kosher salt. Process until the crackers are the texture of sand, about 15 seconds. Add the butter and pulse to combine, 6 to 8 (1-second) pulses. (If using graham cracker crumbs, just stir 1 1/2 cups crumbs and the sugar, salt, and butter together in a medium bowl.)

Step 5

Transfer the mixture into the baking pan. Use the bottom of a measuring cup or your fingers to press the crumbs into an even layer, making sure to reach the edges and fill the corners.

Step 6

Reserve and refrigerate 1/2 cup of the frozen whipped topping for serving. Add the remaining 2 1/2 cups whipped topping into the lemon mixture and gently whisk until no white streaks remain. Pour over the crust and spread into an even layer. Refrigerate until set, 2 to 3 hours.

Step 7

When ready to serve, grasp the excess parchment and transfer the slab onto a cutting board. Cut the slab into 9 squares, dipping the knife in hot water and drying it between each cut for the cleanest-looking pieces. Top each square with a dollop of the reserved whipped topping.