Start by preheating your oven to 200°F. Line a baking sheet with parchment paper. This low temperature is key to drying the meringues slowly without browning them.
In a large bowl, beat egg whites on high for 30 seconds until foamy. Reduce speed to medium and add vinegar and cornstarch, then continue beating until soft peaks form. This ensures your meringue will hold its shape.
Slowly add the sugar in a steady stream while mixing. Increase the speed to high and beat until stiff peaks appear, about 4–5 minutes. This creates that signature glossy, airy texture.
Divide the meringue if using multiple Jello flavors. Add the Jello powder to each portion and beat briefly until blended. The Jello adds flavor and a fun tint to the cookies.
Spoon or pipe meringues onto parchment-lined baking sheets. Bake for 90 minutes, then turn off the oven and leave cookies inside for several hours or overnight to dry completely. This step ensures a perfectly crisp cookie.