Blend together the butter, cheese and egg yolk in a food processor until smooth.
Combine the dijon, dry mustard and mustard seeds in a small bowl and stir. Add to the butter mixture.
Combine the salt, cayenne and flour in a bowl and whisk to combine. Add the dry ingredients to the butter mixture. Put the lid on the food processor and pulse until a soft dough forms. Add the pecans and pulse 2-3 more times until combined. Refrigerate dough for 15 minutes.
Divide the dough into equal portions (2, 3, or 4). Shape and roll the dough into 10" logs. Roll the dough in wax or parchment paper and refrigerate for at least 4 hours or until firm before continuing. (Note: if you're not going to make all the crackers at once, you can freeze the logs well wrapped in parchment and foil or freezer bags for later use.)
Position oven racks in upper and lower thirds of the oven and preheat to 350 degrees. Line baking sheets with parchment paper.
Unwrap logs and cut with a very sharp knife into 1/8" thick rounds. Arrange 1" apart on the baking sheet. Bake in batches, switching positions of the pans halfway through the baking process. Cook a total of 12-15 minutes until lightly golden. Transfer crackers to racks to cool.
Store crackers in an airtight container for up to a week.