Preheat the oven to 350 degrees F. Lightly spray 12 cups of a nonstick mini muffin pan with cooking spray.
For the crust: Place the graham crackers in a food processor and pulse until finely ground. Add the sugar and salt and pulse to combine. With the motor running, drizzle in the butter and process until the mixture resembles damp sand. Divide the mixture evenly among 12 cups of the prepared muffin tin (about 1 1/2 tablespoons for each cup) and use a shot glass or your fingers to press it evenly into the bottom and up the sides. Bake until browned and firm to the touch, about 10 minutes. Set aside while you prepare the filling. (If the crusts fell during baking, you can reform them while they are still hot with a lightly greased shot glass or the back of a lightly greased small spoon.)
For the filling: Beat together the yolks and lime zest in a large mixing bowl with an electric mixer on high speed until the yolks have lightened in color and thickened slightly, about 1 minute. Add the condensed milk and lime juice and beat until combined. Divide the mixture evenly among the tartlet crusts in the muffin pan, filling just below the top of each crust. Bake just until the filling has set, about 6 minutes. Place the pan on a wire rack to cool for about 30 minutes. Remove tarts from the pan (a small offset spatula works well here) and refrigerate until cold, at least 1 hour.
Just before serving, beat the cream, confectioners’ sugar and vanilla in a large bowl with an electric mixer until soft peaks form. Dollop the whipped cream on top of the tartlets and sprinkle with the lime zest.