Meanwhile, in a standing mixer, combine the egg whites and cream of tartar. Mix until frothy.
Continue to mix on medium-low speed, adding sugar slowly, alternating between the two types of sugar, 1 Tablespoon at a time. Your final Tablespoon should end up being powdered sugar.
Continue to mix until stiff peaks form.
Add leaf green gel food color, 1-2 drops at a time, until you achieve your desired shade.
Scoop meringue into the piping bag and pipe dollops of the meringue on top of each other.
Keep leftover meringue to use later to "glue" your edible eyes to the cookies.
Bake for 1 hour and 25 minutes at 225°F.
Once your slime monsters are done baking, remove from oven and allow to cool enough to handle. Use the remaining meringue you set aside earlier to attach edible eyes to the slime cookies.
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Notes
*Nutritional information does not contain edible eyeballs!Note: Nutrition information is a rough estimate only; actual values will vary based on the exact ingredients used and amount of recipe prepared.