Melt the butter in the skillet. Add the leeks and cook over moderate heat, stirring, until softened, about 3 minutes. Add the garlic and cook over moderately low heat until golden, about 3 minutes. Add the wine and saffron and simmer for 2 minutes. Add the stock and bay leaf and bring to a simmer over moderately high heat. Return the chicken breasts to the skillet, skin side up and simmer over moderately low heat until just cooked through, about 10 minutes. Transfer the chicken to a warmed platter.