Line an 8x8 (or 9x9 or 9x13) baking dish with parchment paper.
Melt butter, bittersweet chocolate chips, sugar, corn syrup and cocoa powder together in a medium saucepan over low heat. Turn heat off.
Stir in graham cracker pieces until fully coated.
Stir in marshmallows, chocolate chips, and nuts coating completely after each addition.
Gently press mixture into prepared baking dish and chill for 2 hours.