a 2 1/2-inch-high and 6 1/2-inch-diameter cereal bowl
Melt the marshmallows and butter in a medium pot over low heat, stirring constantly. Remove from the heat, add the cereal and mix. Let cool slightly.
Spray a cereal bowl (2 1/2 inches high and 6 1/2 inches in diameter) with nonstick cooking spray and pour in the marshmallow mixture. Spray your hands with the cooking spray and press the mixture into the bowl and up the sides to create a bowl shape. Set aside.
Microwave the chocolate in a microwave-safe bowl for 30 seconds, stirring occasionally. Continue heating for 10 seconds at a time until the chocolate is completely melted; be careful not to burn it.
Pour the chocolate into the cereal bowl. Use a spatula to spread the chocolate up the sides. Let the bowl set at room temperature for 5 to 10 minutes.