1. Toast fennel seeds in a dry skillet over medium heat, shaking pan continuously, until aromatic, about 4 minutes. Transfer to a spice grinder; grind until seeds are no longer coarse. Repeat with cumin seeds.

2. In a small mixing bowl, stir ground fennel seeds into 1/2 cup coarse salt; transfer to a small serving bowl. Wipe out mixing bowl, then mix ground cumin seeds into 1/2 cup coarse salt; transfer to another small serving bowl. Wipe out mixing bowl, then stir paprika and cayenne pepper into 1/2 cup coarse salt; transfer to a third serving bowl.

3. Arrange radishes on a serving platter. Pour the remaining 1/2 cup salt into another serving bowl. Transfer softened butter to a slightly larger bowl. Serve.