Line two 24 count mini muffin pans with paper liners.
In a large mixing bowl, whisk together water, oil, and egg until well combined. Add brownie mix and stir until well blended. Scoop 1 tablespoon brownie batter into each of the paper-lined muffin cups.
Bake brownies 14 minutes until the brownies are firm to the touch.
Put 4 miniature marshmallows in the center of each brownie. Return to the oven, and bake until marshmallows begin to puff, 1 to 2 minutes. Cool in pan 5 minutes. Transfer brownies to a wire rack to cool before frosting.
Put the chocolate in a small glass bowl and set aside. In a small microwave-safe bowl, combine heavy cream and corn syrup; microwave heavy cream just until it starts to boil, about 30 seconds.
Pour over the chocolate; set aside for 3 minutes. Whisk until chocolate is melted and smooth. If necessary, microwave an additional 10 seconds at 50% power.
Drizzle a spoonful of the glaze on top of the marshmallow. Top with M&M Baking Bits or other colorful candies.