In a mixing bowl, combine Gruyère cheese, sharp white cheddar, cream cheese, Dijon mustard, egg yolk, chives, black pepper, and salt. Mix until smooth.

On a floured surface, roll out the puff pastry into a rectangle. Spread half of the cheese filling on the pastry.

Roll the pastry tightly from one long edge to form a log. Use water to seal the edge.

Wrap the logs in plastic wrap and chill in the refrigerator for 30 minutes to 3 hours.

Slice the logs into 1/2-inch rounds and place on a lined baking sheet. Brush with egg wash.

Preheat the oven to 400°F (200°C) and bake for 15 to 20 minutes until puffed and golden.

Allow to cool slightly before serving warm.