Set the cubed squash in a steamer insert and set over a pot of boiling water. Cover and steam until tender, 8-10 minutes. Set aside to cool.
In a small bowl, whisk together the vinegar, mustard, maple syrup, and chipotle chile, and a pinch of salt and pepper. While whisking, drizzle in the olive oil, and whisk until emulsified.
In a large bowl, combine the watercress, black beans, and scallions. Fold in the butternut squash, and add the dressing. Toss gently to coat. Taste, and adjust the seasoning if needed. Serve.