In a mixing bowl, combine the cold butter cubes and flour. Using your fingertips, rub the butter into the flour until the mixture resembles coarse crumbs.
Add the egg yolk, Parmesan cheese (except for the 2 tablespoons set aside for topping), ground nutmeg, mustard powder, and dried chives to the bowl. Mix until well combined and use your hands to bring the dough together into a ball.
Wrap in cling film and refrigerate for 30 minutes. (Note 6)
Preheat your oven to 200°C (400°F) and line a baking sheet with parchment paper.
Lightly flour a clean work surface and turn the dough out onto it. Roll the dough into a rectangular shape, about 6mm / ¼ inch thick.
Using a knife or a pizza cutter, cut the dough into thin strips, about 1.5cm or ½ inch wide and 10cm/4 inches long.
Carefully transfer the cut cheese straws onto the prepared baking sheet, leaving a small gap between each straw to allow for spreading during baking.
Brush the tops of the cheese straws with egg white to give them a golden colour and a nice shine.
Sprinkle on the extra Parmesan.
Bake in the middle of the oven for 10 minutes until golden.
Remove and cool on the tray for 5 minutes then transfer to a wire rack to cool completely.
Serve the cheese straws as a delicious snack or appetiser or store in an airtight container for up to 4 days.