PREHEAT oven to 200F. Line a large rimmed baking sheet with foil. Set a wire rack on prepared sheet. Spray rack with oil.
PAT wings very dry with paper towels, then transfer to a large bowl. Sprinkle baking powder and salt over thighs. Toss to coat completely. Arrange thighs, skin-side up, in a single layer on prepared rack. Place wings in centre of oven, then increase oven temperature to 400F.
BAKE until golden and crispy, 50 to 60 min. Remove sheet from oven and let stand until cool enough to touch, about 5 min.
COMBINE maple syrup with water, vinegar, espresso powder, cocoa powder and salt in a small saucepan. Boil, stirring occasionally, until sauce starts to thicken and turn syrupy, 12 to 15 min.