1. In a small skillet, toast the sesame, cumin, nigella and yellow mustard seeds over low heat until fragrant. Transfer the seeds to a mortar and let cool. Add 1/2 teaspoon of salt and grind to a coarse powder.
2. Preheat the oven to 300°. Put the garlic in a glass baking dish, cut sides up. Dot the heads with 1 1/2 tablespoons of the butter and season with salt and pepper. Cover tightly with foil and bake for about 1 hour, or until the garlic is very soft. Set aside the 4 nicest garlic halves. Squeeze the rest of the cloves into a coarse strainer set over a bowl and press the garlic through with a spatula.
3. Increase the oven temperature to 400°. Set the poussins in a shallow roasting pan, breasts up. Rub the poussins with the remaining 3 tablespoons of butter and season with salt and pepper. Roast in the upper third of the oven for 1 hour, or until golden brown; baste occasionally. Toward the end of cooking, tilt the birds to allow the juices to run into the roasting pan.
4. Transfer the poussins to a warmed platter. Scrape the pan juices into a small saucepan and skim off the fat. Set the roasting pan over 2 burners on moderate heat. When the pan starts smoking, add the orange juice and the pan juices and simmer, scraping up the browned bits from the bottom. Strain the sauce into the small saucepan and simmer for 1 minute. Stir in the garlic puree and season with salt and pepper.
5. Set the birds on warmed plates and sprinkle generously with the spice dust. Garnish each plate with a reserved roasted garlic half and serve. Pass the garlic sauce at the table.
Serve With: Steamed jasmine rice.
Notes: Tiny black nigella seeds, which have a slightly oniony flavor, are available at specialty shops and Indian groceries.