In a medium saucepan melt the butter over a moderate heat. Whisk in the flour and cook for 2 minutes, whisking constantly. Pour in the chicken stock and cook, whisking occasionally, until thickened, about 1 minute. In a small bowl whisk together the egg yolks and crÅme fraöche. Stir the cream mixture into the saucepan and heat gently, but do not allow to boil. Season with salt, pepper and fresh lemon juice.
