Prepare two large baking sheets with parchment paper. Preheat oven to 275 degrees.
Prepare chocolate first, melting in a double broiler or in the microwave, so it can cool as the meringue is whipped. To melt chocolate, place in a small microwave-proof bowl. Microwave for one minute, then stir. Continue to microwave in 30 second increments, stirring after each, until chocolate is nearly smooth, then stir. A butter knife works well for stirring. If chocolate is melted first and not allowed to get too hot, it should be cool by the time the meringue is whipped. Add the espresso powder to the chocolate if desired.
Place egg whites in a stand mixer bowl fitted with a whisk and whip until frothy. Mix sugar and cornstarch together. Gradually add sugar mixture to the beaten egg whites and continue beating until mixture is glossy and forms stiff peaks. Add in vinegar and vanilla or peppermint extract if using, and beat until just mixed in, scraping as necessary.
Drizzle about a third of the cooled chocolate over the meringue, using a butter knife to make one or two cuts through the top third of the meringue. Simple is better, just a quick cut or two through any big, huge dollops of chocolate is all that is needed.
Working quickly, using a two-tablespoon scoop, scoop in a manner that starts just before a drizzle of chocolate and ends after (So chocolate will show more prominently at the top.) Make about 10, then repeat the drizzling and scooping.
Place fairly closely on baking sheet (you’ll want about 17 on a sheet, so 3 across, five down, leaving room at the bottom edge for an extra two to be squeezed in.)
Bake for 35 to 40 minutes until tops are set for a chewier meringue. Add an additional few minutes for a meringue that is crispier all the way through. Remove from the oven and cool to room temperature. Store in an airtight container after completely cool.
Keywords: Chocolate, cocoa, Cookies, Desserts, egg whites, meringue, meringue cookies, peppermint