In a saucepan, melt 1 tablespoon of the butter. Add the bacon and cook over moderate heat until the fat has been rendered, about 4 minutes. Add the leek and cook, stirring occasionally, until softened, about 4 minutes. Add the Cognac and carefully ignite it. When the flames subside, add the chicken stock and chestnuts and bring to a boil. Simmer over moderate heat for 3 minutes. Transfer to a blender and puree. Return the chestnut sauce to the saucepan and season with salt and pepper.

In a large skillet, heat the vegetable oil. Season the scallops with salt and pepper. Add the sage leaves to the skillet and cook over moderately high heat, turning once, until crisp, about 1 minute. Using a slotted spoon, transfer the sage to a plate.

Add the scallops to the skillet and cook over high heat until starting to brown, about 1 minute. Reduce the heat to moderately high and cook the scallops until they're well glazed, about 1 minute. Turn the scallops and add the remaining 1 tablespoon of butter to the skillet. Continue cooking the scallops, basting with the butter, until browned and just cooked through, about 2 minutes longer.

Reheat the chestnut sauce and pour it into shallow bowls. Arrange the scallops in the chestnut sauce and garnish with the fried sage leaves. Serve right away.