Use a fork to mix the ingredients until the mixture resembles wet sand. In a separate bowl, whisk together the key lime juice and sweetened condensed milk. Set aside.
In a third mixing bowl, use an electric hand mixer to whisk the heavy cream, powdered sugar, and vanilla together until the cream forms stiff peaks.
Press a few spoonfuls of the graham cracker mixture into 4 separate 8 oz wide mouth mason jars. Top the graham cracker layer with the key lime mixture, leaving about ¼ of the jar empty.