- Portion size 64 servings
Preheat oven to 350°F (180°C).
On board, chop bittersweet and unsweetened chocolates. Place chocolate and butter in large heatproof bowl over slightly smaller saucepan of hot (not boiling) water. Melt, stirring with metal spoon, until about three-quarters melted. Remove from heat and let finish melting; stir until smooth. Whisk in sugar, then eggs, 1 at a time, and vanilla.
In separate bowl, whisk together flour, baking powder and salt. Pour over chocolate mixture; stir just until combined. Sprinkle chocolate chips, marshmallows and walnuts over batter; stir to combine.
Drop by heaping tablespoonful (15 mL) about 2 inches (5 cm) apart onto parchment paper-lined or ungreased rimless baking sheets. Bake, 1 sheet at a time, in centre of oven for 10 to 12 minutes or until firm around edges but still soft in centre. Let firm up on pan on rack for 5 minuts; transfer cookies to rack to finish cooling.