Melt the 12 ounces of bittersweet chocolate pieces in a microwave-safe dish or over a gently simmering double-boiler. Let cool but not harden. Add the vanilla.
Preheat the oven to 325 degrees F. Line three baking sheets with parchment paper.
Place the salt and vinegar (or lemon juice) in a clean mixer bowl for a stand mixer. Use a paper towel to wipe off any possible grease. Use the paper towel to wipe the blades of the whisk attachment as well. Do not rinse. This procedure will ensure that the egg whites will whip to their full potential.
Beat the egg whites on medium speed until foamy, about 1 minute. Increase the speed to high. Slowly add the sugar in a steady stream and continue beating until stiff peaks form, 4 to 5 minutes. (The bowl can be turned upside down without the meringue falling out.) Add a third of the meringue to the melted chocolate and mix well to lighten. Gently but thoroughly fold the melted chocolate into the meringue, and then fold in the remaining two ounces chopped bittersweet chocolate and the walnuts.
Fill a large resealable plastic bag with the meringue and snip off the bottom corner, or use a pastry bag. Pipe the meringue into 1 1/2-inch rounds 2 inches apart on the prepared baking sheets.
Immediately place in the oven and bake about 10 minutes. The cookies will develop a shiny crust but will still be gooey inside.
Let rest on the baking sheets for 10 minutes before removing to a rack to cool completely. Store in an airtight container for up to 2 days.