In a small mixing bowl, combine sugars, cinnamon and salt; mix well. Pour mixture onto a small rimmed plate; set aside. Line a rimmed baking sheet with parchment paper; set aside. Working 4-6 crackers at a time, use a pastry brush to evenly brush one side of each cracker with butter from edge to edge. Press the buttered side of each cracker into sugar mixture; place crackers on sheet pan buttered-side up, sides touching. Watching carefully so crackers don't burn, bake in a 375° oven until sugars just start to melt and stick together (crackers will still have a sandy, sugary top) and crackers are barely darkened on the edges (about 5 min.). Remove pan from the oven; let crackers rest until cool (about 30 min.). Store at room temperature for up to a week.