In a medium skillet, melt the butter. Add the pecans and cook over medium-low heat, stirring frequently, until the nuts turn a deeper brown, 3 to 5 minutes. Sprinkle with 2 teaspoons of the sugar and the cayenne and stir just until the sugar dissolves. Turn the nuts onto parchment or wax paper to cool.
In a medium bowl, whisk together the olive oil, vinegar, mustard, salt and the remaining tablespoon of sugar.
In a large serving bowl, combine the greens, avocado, mango and pecans. Toss with the dressing and season to taste with salt.