Spray or grease a 8 x 8 or 9 x 9 inch pan. Set aside.
Place evaporated milk, sugar and butter in a large, deep saucepan. Bring to boil over medium heat, stirring constantly. When mixture begins to boil, reduce heat to medium-low and cook for 10 minutes, stirring occasionally. Mixture will foam and rise…you need a DEEP saucepan as the volume will double when boiling…this is important!!!
Add the flour, chocolate chips, walnuts and graham cracker crumbs. Stir until well combine.
Pour in prepared pan and let cool. Pan may also be placed in the refrigerator to speed the cooling process (about 30 minutes in refrigerator).