Preheat the oven to 325 degrees . Line an 8-inch square baking pan with foil, letting two opposing edges hang over at least an inch or so. Grease the foil with cooking spray.
For the crust, process the animal crackers, sugar and salt together to fine crumbs in a blender or food processor. Drizzle the melted butter over the crumbs and pulse to incorporate. Sprinkle the mixture into the prepared pan and press the crumbs into an even layer. Bake the crust until golden, about 15-18 minutes.
For the filling, while crust bakes, in a medium bowl, stir the cream cheese, zest and salt together until combined. Whisk in the sweetened condensed milk until smooth (you can use a handheld electric mixer, if you want). Whisk in the egg yolk and juice until combined.
Pour the filling evenly over the crust. Bake the bars until the filling is set and the edges just slightly begin to pull away from the sides of the pan, 15-17 minutes.
Let the bars cool completely in the pan set on a wire rack, about 2 hours. Then cover the pan with foil and refrigerate until chilled, about 2 hours. Remove the bars from the pan using the foil overhang and cut into squares.