Our spicy garlic mayonnaise, a customary accompaniment, includes cayenne.
Preheat oven to 325 degrees. Cut baguette into sixteen 1/4-inch-thick slices and three 1/2-inch-thick slices. Cut 1/2-inch-thick slices into cubes; soak in broth. Arrange slices on a baking sheet. Brush one side with oil. Toast in oven about 10 minutes. Using a mortar and pestle, pound and stir garlic, salt, saffron, and cayenne into a paste. Squeeze broth from cubes (reserve broth); add cubes to mortar. Add yolk; pound and stir. While stirring, add oil in a slow, steady stream until emulsified. If needed, thin with reserved broth. Serve on croutons.
Raw eggs should not be prepared for pregnant women, babies, young children, the elderly, or anyone whose health is compromised.
Rouille can be made in a food processor; with machine running, add oil last.