Step 1: Preheat and Prepare

Preheat oven to 350°F. Line baking sheets with parchment paper.

Step 2: Cream Butter and Sugars

Beat together unsalted butter, granulated sugar, and brown sugar for 2-3 minutes until light and fluffy.

Step 3: Add Wet Ingredients

Mix in egg yolk and vanilla extract until fully incorporated.

Step 4: Combine Dry Ingredients

Fold in flour, salt, and nutmeg until just combined. Do not overmix.

Step 5: Shape Dough

Roll dough into 1-inch balls. Place on prepared baking sheet, spacing 2 inches apart.

Step 6: Create Thumbprints

Gently press your thumb into the center of each dough ball to create an indent about 1/2 inch deep.

Step 7: Bake

Bake for 10-12 minutes until edges are lightly golden.

Step 8: Fill with Chocolate

Remove from oven and immediately fill each warm indent with chocolate chips. Let melt for 1-2 minutes.

Step 9: Cool

Cool on baking sheet for 10 minutes, then transfer to wire rack. Allow chocolate to fully set before serving.