Cook the bacon in a large cast-iron pan in the oven or on the stovetop until browned and crisp and the fat has rendered out, about 10 minutes. Set the bacon aside on a cutting board to let cool. Increase the oven temperature to 450 degrees F.
Arrange the Brussels sprouts in a single layer in concentric circles, cut-side down, in the bacon fat in the pan. Sprinkle with salt and pepper. Cook over medium-high heat until the sprouts begin to caramelize, about 4 minutes . When they begin to brown, transfer the pan to the oven and roast until charred and tender, about 15 minutes.
Return the pan to medium heat. Transfer the sprouts to a bowl, leaving some fat behind. Add the shallots and cumin to the pan and cook, stirring often, until the shallots soften and the cumin smells fragrant, 1 to 2 minutes. Add the maple syrup, mustard and salt and pepper to taste, and whisk until bubbly. Add the sprouts to the sauce and remove the pan from the heat. Add the chives and stir to coat the sprouts in the sauce. Transfer to a serving plate.