Spray an 8-inch square baking dish lightly with nonstick cooking spray. Combine the butter, granulated sugar and 1 1/2 cups of the crushed graham crackers in a medium bowl. Press the mixture into the prepared baking dish and freeze for 15 minutes to set.
Combine the cream cheese, confectioners' sugar, salt, lime juice and two-thirds of the zest in a large bowl. Beat with an electric mixer on medium-high speed until fluffy, about 3 minutes. Fold in 3/4 cup whipped topping to lighten. Pour the filling over the crust and smooth with an offset spatula. Refrigerate for 15 minutes to partially set.
Meanwhile, mix the instant pudding powder, milk and key lime juice in a bowl. Whisk until thick, about 2 minutes. Pour over the filling. Top with the remaining whipped topping, cover and refrigerate for 2 hours to set.
Top with the remaining graham cracker crumbs and lime zest before serving.
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