Preparation Preheat oven to 350°F. Line 3 large rimmed baking sheets with parchment paper. Whisk flour, sugar, salt, and baking soda in large bowl. Whisk eggs, buttermilk, molasses, and vanilla extract in medium bowl; whisk in butter. Add egg mixture to dry ingredients, stirring until dough is evenly moistened. Stir in chocolate chips, marshmallows, and nuts. Drop cookie dough by rounded tablespoonfuls onto prepared sheets, spacing about 3 inches apart (about 12 cookies per sheet). Bake cookies, 1 sheet at a time, until golden brown, dry to touch, but still slightly soft, about 15 minutes. Let cookies cool on sheets 10 minutes. Transfer cookies to racks and cool (cookies will firm up). DO AHEAD: Can be made 2 days ahead. Store in airtight container at room temperature. TEST-KITCHEN TIP:No buttermilk? In a pinch, use 1/2 cup milk soured with 1/2 tablespoon lemon juice. Let stand at room temperature 15 minutes.
Preparation Preheat oven to 350°F. Line 3 large rimmed baking sheets with parchment paper. Whisk flour, sugar, salt, and baking soda in large bowl. Whisk eggs, buttermilk, molasses, and vanilla extract in medium bowl; whisk in butter. Add egg mixture to dry ingredients, stirring until dough is evenly moistened. Stir in chocolate chips, marshmallows, and nuts. Drop cookie dough by rounded tablespoonfuls onto prepared sheets, spacing about 3 inches apart (about 12 cookies per sheet). Bake cookies, 1 sheet at a time, until golden brown, dry to touch, but still slightly soft, about 15 minutes. Let cookies cool on sheets 10 minutes. Transfer cookies to racks and cool (cookies will firm up). DO AHEAD: Can be made 2 days ahead. Store in airtight container at room temperature. TEST-KITCHEN TIP:No buttermilk? In a pinch, use 1/2 cup milk soured with 1/2 tablespoon lemon juice. Let stand at room temperature 15 minutes.