1. Toast walnuts on a baking sheet at 350º for 6 minutes
2. Chop roughly and set aside
3. Line an 8" square baking pan with foil, pressing the foil into the corners and leaving a 2" overhang on two opposite sides
4. Melt the chocolate and butter in a large heatproof bowl set over a pan of barely simmering water, about 5 minutes, stirring constantly.  When the chocolate is almost fully melted, remove the bowl from the heat and stir to melt the remaining pieces.  Let cool at room temperature for 5 minutes
5. Stir in 1 cup of the marshmallows, 1 cup of the walnuts and all graham cracker pieces. Spread the mixture into the prepared pan. Sprinkle with remaining ½ cup of marshmallows and ¼ cup of walnuts, pressing down lightly to adhere
6. Let stand at room temperature until the chocolate sets, about 2 hours, or 1 hour in refrigerator
7. Grab foil overhang to lift the set mixture out of the pan, and place on a cutting board. Peel the foil from the sides. Using a bench scraper or a sharp knife, cut into small squares (20-30 depending on desired piece size)
8. Store, layered between sheets of wax paper in an airtight container for up to 1 week, or refrigerate up to 3 weeks

Adapted from "Christmas Cookies: 50 Recipes to Treasure for the Holiday Season"