Preheat oven to 410°F (210°C).

Cream cold butter and sugars until combined but not overly light.

Add eggs and vanilla, mixing just until incorporated.

Stir in dry ingredients until just combined.

Fold in chocolate chips, marshmallows, and walnuts.

Form into large, rough balls (about 6oz each) and place on a baking sheet.

Bake for 10-12 minutes. The outside should be set but the inside still gooey.

Let cool on the tray for at least 15 minutes.