Mix together graham cracker crumbs, 1/4 cup sugar, melted butter or margarine in a bowl until well blended. Press mixture into prepared pan.
Bake at 375℉ for 7 minutes. Cool completely.
Key Lime Filling
Combine the milk, 1 ⅔ sugar and salt in a large, heavy saucepan and cook over medium heat until mixture comes to a boil; stirring constantly. Once mixture begins to boil, cook it for 8 minutes, stirring constantly.
Remove from heat and stir in the marshmallows, chocolate, food coloring, lime rind and lime and mix well until mixture is melted and smooth. Pour into prepared pan and cool completely. Cut into 1 inch squares to serve.
Notes
If you’re worried about your quality of chocolate, or your chocolate seizing up, I would add in everything else before adding in the chocolate. That way, the mixture cools off just a little bit, but it’s still more than warm enough to melt the chocolate.