Preheat the oven to 400ºF. Wash the beets and place them on a baking sheet. Cook until tender, 60-75 minutes. Let cool until cool enough to handle, then gently rub off the skins. Cut each beet into 6-8 pieces.
Whisk together the olive oil, vinegar, cocoa powder, honey, salt, and pepper. Toss the salad greens in the vinaigrette and divide onto 6 salad plates. Top the salad with the beets, oranges, goat cheese, and nuts. Serve immediately.