1 stick (1/2 cup) unsalted butter, softened
3/4 lb sharp Cheddar, coarsely grated (preferably in a food processor)
1 large egg yolk
1 cup all-purpose flour
1 teaspoon dried mustard
3/4 teaspoon salt
1 teaspoon cracked black pepper
1 teaspoon caraway seeds
1 teaspoon nigella seeds
Blend together butter, cheese, and yolk in food processor until smooth. Add flour, dried mustard, and salt and pulse until just combined. Transfer dough to a sheet of wax paper and divide into 3 portions (do not clean processor).
Return 1 portion to processor, add pepper, and pulse until combined well, then transfer to another sheet of wax paper. Shape into a 7-inch-long log (1 1/2 inches thick), using paper as an aid, then roll up log in paper and twist ends of paper to close. Make 2 more logs on separate sheets of wax paper in same manner, using caraway seeds for second log and nigella seeds for third log (instead of pepper) and cleaning processor in between batches. Chill logs until firm, about 2 hours.
Put oven rack in middle position and preheat oven to 350°F. Line a large baking sheet with parchment paper.
Unwrap 1 log and cut enough thin slices (about 1/8 inch thick) from it to fill baking sheet, arranging slices 1 inch apart. Bake until edges of crackers are golden, 10 to 12 minutes. Transfer on parchment to a rack and cool slightly, about 15 minutes. Make more crackers in batches with remaining dough.
Serve crackers warm or at room temperature.