5 tablespoons unsalted butter, plus more for the pan
1 (10 1/2-ounce bag) mini marshmallows
1/3 cup unsweetened Dutch-process cocoa
1 teaspoon pure vanilla extract
6 cups crispy rice cereal
Melted white chocolate, for drizzling
Butter a 13-by-9-inch baking dish.
Add the butter, marshmallows, and cocoa powder to a large saucepan. Place over medium heat and stir until melted, about 5 minutes. Stir in the cereal until combined. Press into the buttered baking dish. Drizzle with the melted white chocolate. Let cool, and then cut into bars.