Cream butter and brown sugar in medium bowl. Mix in egg yolk and vanilla then stir in flour and salt to form dough.
Lightly grease bottom of griddle plate. Press dough evenly into griddle pan.
Attach griddle pan to bottom of your George Foreman Evolve grill with grill plate attached on top.
Turn grill on and set temperature for 350°F.
Bake covered 10-12 min. while grill is preheating. DO NOT OVER BAKE.
Open grill cover and place 3 chocolate bars on top of dough. Close cover and let bake an additional minute or until chocolate is soft and spreadable.
Remove griddle pan from grill; place on heat proof surface.
Spread chocolate to cover surface of bars. Sprinkle with nuts.
Cut into 2-inch squares, using a plastic knife or spatula.