Step 1: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
Step 2: In a large, very clean mixing bowl, beat the egg whites with an electric mixer on medium speed until foamy. Add the cream of tartar (if using).
Step 3: Gradually add the granulated sugar, 1 tablespoon at a time, while continuing to beat on high speed. Beat until stiff, glossy peaks form and the sugar is fully dissolved (test by rubbing a little meringue between your fingers – it should feel smooth).
Step 4: Gently fold in the vanilla extract and then the mini chocolate chips (or other desired mix-ins) using a spatula. Be careful not to deflate the meringue.
Step 5: Drop rounded tablespoons of the meringue mixture onto the prepared baking sheets, spacing them about 1 inch apart.
Step 6: Place the baking sheets in the preheated oven. Immediately turn off the oven.
Step 7: Leave the cookies in the oven, undisturbed, overnight (or for at least 6-8 hours), until the oven is completely cool. Do not open the oven door at all during this time.
Step 8: The next morning, remove the crisp, cooled Forgotten Cookies from the oven. Carefully peel them off the parchment paper.
Step 9: Store in an airtight container at room temperature.