For the Mexican chocolate ganache: Put the chocolate in a heatproof bowl. Bring the cream just to a boil in a small pan. Pour immediately over the chocolate and let sit for a minute or 2. Whisk until the chocolate is melted and the ganache is completely smooth. Set aside to cool.
For the ice cream sandwiches: Break the graham crackers in half. Put the candied walnuts on a small plate. Turn the pint of ice cream on its side and use a serrated knife to slice through the carton, creating 6 ice cream discs. Spread some of the Mexican chocolate ganache on each graham cracker half. Add an ice cream disc (remove the carton) to 6 of the graham cracker halves and top with the remaining graham crackers. Roll the edges in the candied walnuts. Serve immediately.