1 ½ cups semisweet chocolate chips (from 12-oz bag)
1 cup mini marshmallows (from 10-oz bag)
Flaky salt, for finishing
⅓ cup white chocolate chips (from 12-oz bag)
Heat oven to 350°F. Line large cookie sheets with parchment paper.
In medium microwavable bowl, add cereal. In small microwavable bowl, microwave 3 tablespoons butter uncovered on high 30–40 seconds or until melted. Stir in 1 tablespoon sugar; microwave about 15 seconds or until sugar is nearly dissolved. Pour butter mixture over cereal, mixing to coat. Microwave uncovered on high 2–3 minutes, stirring every 30 seconds, until glazed and toasted; cool 15 minutes. Place cereal in large resealable food-storage bag, and coarsely crush; set aside.
In small bowl, mix flour, baking cocoa, baking soda, and salt; set aside.
In large bowl, beat softened butter, brown sugar, and ¾ cup sugar with electric mixer on medium speed, or mix with spoon until well combined; scrape side of bowl. Beat in egg until blended. Beat in vanilla. Stir flour mixture into butter mixture until well mixed (dough will be stiff). Stir in crushed cereal, chocolate chips, and marshmallows.
Shape into 18–20 (1 ½ -inch) balls. Place 2 inches apart on cookie sheets. Flatten slightly, reshaping edges if necessary, to make round. Cover balls while waiting to bake.
Baking one cookie sheet at a time, bake 7–9 minutes or until edges are set (centers will be soft). Cool completely on cookie sheet, about 15 minutes. Remove from cookie sheet to cooling rack.
In small microwavable bowl, microwave white cooking chips uncovered on high 30 seconds; stir. If needed, microwave in additional 15-second increments, stirring vigorously after each, until melted and smooth. Transfer to resealable food-storage bag. Cut small corner from bag, and lightly drizzle onto cookies. (Can also use fork to drizzle.) Allow to set, about 15 minutes. Store in an airtight container for up to four days.