Roast and caramelize the cashews: Preheat the oven to 350°. Line the bottom of a sheet pan with parchment paper. Use only whole cashews, not broken pieces. Spread them on the prepared pan in a single layer. Roast the nuts until they are medium brown, about 12 minutes. While the nuts are roasting, put a piece of parchment paper or a nonstick baking liner on a work surface. Measure the sugar and butter and put both of them next to the stove. As soon as the nuts are roasted, place a medium heavy-bottomed pot over medium-high heat. Use an unlined copper pot if you have one. When the pot is hot — test by flicking a drop of water into the pan; it should sizzle and dance across the bottom — immediately add the nuts and the sugar. Cook, stirring constantly with a wooden spoon, until the sugar liquefies and coats the nuts. If they start to smoke, reduce the heat. When only a few specks of sugar remain unmelted, remove from the heat and add the butter. Stir until the nuts glisten and start to separate from one another. Scrape the nuts onto the parchment paper or baking liner and separate them into individual pieces as best as you can. When they are cool enough to handle, carefully break apart any that are stuck together. Let the nuts cool to room temperature. Coat the nuts with chocolate: Chill a large metal bowl in the refrigerator for at least 20 minutes. Line the bottom of a large sheet pan with parchment paper. Sift the cocoa powder into a medium bowl. Put the caramelized nuts in the chilled bowl. Add the tempered chocolate and stir in to coat the nuts with the chocolate. When the chocolate has almost set, after about 5 minutes, sift about half the cocoa powder onto the nuts. Stir until the nuts start to separate, and then toss them with your hands to separate them into individual pieces. Add more cocoa powder if needed to separate them completely. You may have cocoa powder left over. Put the nuts in a sieve, shake them to remove any excess cocoa powder, and then spread them in a single layer on the prepared pan. When the chocolate has set, store the nuts in an airtight container in a cool, dry place, not in the refrigerator, for up to 2 weeks.