How to Make It
Preheat the oven to 425°. Rub the poussins all over with paprika and 1 tablespoon of the butter. Season with salt and pepper. Truss the poussins by simply tying the legs together with string. Set the poussins in a large, shallow metal roasting pan. Roast the poussins in the upper third of the oven for 1 hour, or until nicely browned and the juices from the cavities run clear.
Meanwhile, in a small bowl, mix the remaining 1 tablespoon of butter with the flour until a smooth paste forms.
Transfer the poussins to a large platter, draining their juices into the roasting pan. Pour the pan juices into a glass measuring cup and skim the fat from the surface: reserve 1 tablespoon of the fat.
Set the roasting pan on two burners. Add the chicken stock and bring to a boil over moderately high heat, scraping up the browned bits from the bottom of the pan. Pour the stock into the glass measuring cup.
Heat the reserved 1 tablespoon of fat in a medium saucepan. Add the shallots and cook over low heat, stirring, until softened, about 5 minutes. Add the liquid in the measuring cup and bring to a simmer over moderate heat. Whisk in the anchovies, mustard and flour paste and whisk until the sauce is smooth and slightly thickened, about 3 minutes. Remove the pan from the heat, stir in the herbs and season with salt and pepper. Pour the sauce into a warmed gravy boat.
Cut the strings from the poussins and transfer them to plates. Pass the anchovy-mustard sauce at the table.