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.

Notes:

• Meringue whips better when there’s no trace of fat in the mixing bowl or beaters. Make sure they’re clean and if in doubt, wipe down with a little vinegar.
• Eggs whites should be bought to room temperature or be slightly warm if possible but isn’t strictly necessary. Zap in the microwave for 15 to 20 seconds.
• A two-tablespoon scoop will give a cookie that is about 2 inches across & 2 inches high. Work quickly or the last of them will be flatter and spread.
• These are done when the tops are firm, but may be baked a few minutes longer if you’d like a crisper, less chewy cookie.
• May be piped. Dollop a cup of the meringue, layer with chocolate, add another cup, then layer with chocolate. Repeat till meringue is gone.
• For a smoother, less dry chocolate in the cookie, mix the melted chocolate with 2 tablespoons of heavy cream.