Preparation Toast fennel seeds in a small heavy dry skillet over medium heat, shaking skillet often, until fragrant, 1-2 minutes. Let cool. Finely grind fennel seeds and whole black peppercorns in a spice mill; transfer to a small bowl. Stir in kosher salt and cayenne pepper. DO AHEAD: Can be made 1 month ahead. Store airtight at room temperature.
Preparation Toast fennel seeds in a small heavy dry skillet over medium heat, shaking skillet often, until fragrant, 1-2 minutes. Let cool. Finely grind fennel seeds and whole black peppercorns in a spice mill; transfer to a small bowl. Stir in kosher salt and cayenne pepper. DO AHEAD: Can be made 1 month ahead. Store airtight at room temperature.