In a large skillet, melt the butter in the olive oil over moderate heat. Add the curry powder and stir until fragrant, about 45 seconds. Add the carrots and cook until just tender, about 7 minutes. Add the leeks and cook, stirring occasionally, until beginning to brown, about 4 minutes. Add the stock and bring to a boil, then simmer over low heat for 4 minutes. Season with salt and pepper.