Position oven racks in the upper and lower thirds of the oven and preheat the oven to 350 degrees. Line two large baking sheets with parchment paper.
Using the small bowl of a stand mixer on medium-high speed, beat the egg whites with the salt until they form soft peaks. Reduce speed to medium-low and add the sugar by the teaspoonful, blending well after each addition. When about half of the sugar has been incorporated, add the vanilla extract and lemon juice or cider vinegar, then add the rest of the sugar. Increase speed to high just until the mixture forms stiff peaks. Remove the bowl from the mixer. Add the melted chocolate, folding it in with a spatula as lightly as possible and leaving streaks of white in the batter. Gently fold in the walnuts.
Drop rounded teaspoonfuls of the batter onto the prepared baking sheets, about 11/2 inches apart. Bake for 10 minutes.
Use your fingers to lift the cookies off the parchment and carefully turn them on their sides to cool completely. Store in an airtight container for up to 3 days.