Make the spice blend: Toast the cinnamon stick, both peppercorns, cumin, coriander, allspice, cloves and cardamom in a small dry skillet over medium heat until fragrant, 2 minutes; let cool. Grind in a spice grinder or mini food processor with the ginger, turmeric, cayenne, nutmeg and 1 teaspoon salt. Store in an airtight container for up to 2 weeks; use leftover spice blend as a rub for meat or fish.
Make the couscous: Heat the butter, 1 1/2 teaspoons spice blend and the saffron, if desired, in a large skillet over medium-high heat until lightly toasted, about 1 minute. Add the couscous and stir until toasted, about 1 minute. Stir in the warm broth, remove from the heat, cover and set aside until the couscous is plump, about 7 minutes. Add the lemon juice and fluff with a fork. Transfer to a bowl and sprinkle with 1/2 teaspoon spice blend.