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1
Melt the 12 ounces of bittersweet chocolate pieces in a microwave-safe dish or over a gently simmering double-boiler.
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2
Let cool but not harden.
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3
Add the vanilla.
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4
Preheat the oven to 325 degrees F. Line three baking sheets with parchment paper.
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5
Place the salt and vinegar (or lemon juice) in a clean mixer bowl for a stand mixer.
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6
Use a paper towel to wipe off any possible grease.
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7
Use the paper towel to wipe the blades of the whisk attachment as well.
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8
Do not rinse.
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9
This procedure will ensure that the egg whites will whip to their full potential.
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10
Beat the egg whites on medium speed until foamy, about 1 minute.
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11
Increase the speed to high.
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12
Slowly add the sugar in a steady stream and continue beating until stiff peaks form, 4 to 5 minutes.
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13
(The bowl can be turned upside down without the meringue falling out.)
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14
Add a third of the meringue to the melted chocolate and mix well to lighten.
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15
Gently but thoroughly fold the melted chocolate into the meringue, and then fold in the remaining two ounces chopped bittersweet chocolate and the walnuts.
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16
Fill a large resealable plastic bag with the meringue and snip off the bottom corner, or use a pastry bag.
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17
Pipe the meringue into 1 1/2-inch rounds 2 inches apart on the prepared baking sheets.
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18
Immediately place in the oven and bake about 10 minutes.
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19
The cookies will develop a shiny crust but will still be gooey inside.
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20
Let rest on the baking sheets for 10 minutes before removing to a rack to cool completely.
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21
Store in an airtight container for up to 2 days.
