Preheat oven to 225°F (110°C), line two baking sheets with parchment or silicone mats, and position racks in the upper and lower thirds.

Beat egg whites and cream of tartar in a clean, dry bowl on medium speed until soft peaks form and gently curl.

Increase to high speed and add sugar one tablespoon at a time, beating until stiff, glossy peaks form and the mixture feels smooth.

Add Jello powder and beat just until fully combined and evenly colored.

Pipe or spoon small mounds onto prepared baking sheets, spacing them about 1 inch apart.

Bake for 1½ to 2 hours until completely dry, then turn off the oven and let cookies cool inside for 1 hour.

Remove cookies and store in an airtight container at room temperature before serving.