Step 1

Whisk salt and sugar in a medium bowl to combine. Evenly spread out half of salt mixture in an 8x8" glass baking dish. Using the back of a tablespoon, create 4 depressions in salt mixture, spacing evenly. Carefully place an egg yolk in each depression. Gently sprinkle remaining salt mixture over yolks and tightly wrap dish with plastic. Chill 4 days.

Step 2

Preheat oven to 150°. Brush salt mixture off each yolk, then carefully rinse under cold water to remove any remaining salt (yolks will be semi-firm, bright, and translucent). Gently pat dry with paper towels.

Step 3

Generously coat a wire rack set inside a rimmed baking sheet with nonstick spray; place yolks on rack. Dry out in oven until opaque and texture is like a firm Gruyère cheese, 1½–2 hours. Let cool. (Alternatively, if your oven doesn’t go that low, you can dry out eggs in an unheated oven for 2 days.)

Step 4

Finely grate cured egg yolks over soups, pastas, or salads as you would a hard cheese.

Step 5

Do Ahead: Yolks can be cured 1 month ahead. Place in an airtight container and chill.