I first made these rochers (aka misshapen meringues) because I was missing the rochers at Tartine Bakery in San Francisco. Since then, they've gone into regular rotation whenever sweets are called for, and I'm even adding them to this year's Christmas cookie collection for a little something different.
Heat oven to 300°F and arrange racks in the upper and lower thirds of the oven. Combine sugar and cocoa powder in a small bowl and whisk until no lumps remain. Place egg whites in an impeccably clean bowl and beat on low speed until frothy, and little white uniform bubbles appear, about 2 minutes. Add cream of tartar and increase speed to medium, continuing to beat until the mixture doubles in volume and there are soft peaks when you remove the whisk from the bowl, about 1 minute more.
Superfine sugar is smaller grain than regular white sugar and dissolves better in desserts like this. If you cant find it, go ahead and use granulated sugar and know it may be a tad grainy in texture.
Add the sugar mixture 1 tablespoon at a time, until firm, glossy peaks form, about 2 to 3 minutes. Add vanilla and salt and allow mixer to turn a few more times just until the ingredients are incorporated. Test if the meringue is adequately whipped by pulling the whisk out of the bowl and turning the whisk upside down. If the meringue leaves a peak that stands up but gently folds back on itself at the end, its ready.
Remove the bowl from the mixer stand, and quickly, gently, fold in the cacao nibs and walnuts. Using two large spoons, immediately spoon the meringue by heaping tablespoons, nicely spaced apart, onto the baking sheets. You should be able to fit about 14 meringues on each sheet pan.
Place the baking sheets in the oven. Reduce the temperature to 250°F. Bake until the meringues are somewhat dry to the touch on the outside and very pale golden brown, rotating and switching the baking sheets halfway through, about 40 to 45 minutes total. As the meringues cool they will dry out more and harden on the outside. If, when you open the oven to rotate them, they appear to be cracking or taking on too much color, reduce the temperature by 25 degrees.
When the meringues are ready, remove them from the oven and cool on the pan on a wire rack for at least 20 minutes, or until completely cool.