How to Make It
In a large skillet, toast the ancho chiles over moderately high heat, turning, until fragrant and pliable, about 1 minute. Transfer to a heatproof bowl, cover with 2 cups of hot water and let stand until softened, about 20 minutes. Drain, reserving the soaking liquid.
In a small saucepan, simmer the orange juice over moderately high heat until reduced by half, 3 to 5 minutes. Transfer to a blender and add the stock, dulce de leche, anchos, 1 cup of the chile soaking liquid and onethird of the garlic. Puree until smooth.
Heat the large skillet. Season the duck breasts with salt and pepper and add to the skillet skin side down. Cook over moderate heat, spooning off the fat, until golden and crisp, about 10 minutes. Turn the duck skin side up and add the árbol chile, thyme, 2 tablespoons of the butter, the lemon juice and the remaining garlic to the skillet. Cook, basting the duck occasionally, until medium within, about 8 minutes. Transfer the duck to a carving board and let rest for 5 minutes.
Pour off all but 2 tablespoons of fat from the skillet. Add the chile mixture and bring to a boil. Simmer over moderately high heat, stirring frequently, until just thickened, about 5 minutes. Off the heat, whisk in the remaining 1 tablespoon of butter and the chopped cilantro. Discard the árbol chile and thyme; season the sauce with salt and pepper. Thinly slice the duck across the grain and serve with the sauce and cilantro sprigs.