Step 1

Preheat the oven to 350 degrees. Spray you cookie sheets or line with parchment paper.

Step 2

Beat the sugar and butter together until light and fluffy, about 3 minutes.

Step 3

Add the egg yolks one a time and blend thoroughly before adding another one. Add the vanilla.

Step 4

Whisk together the flour, baking soda, and salt. Add to the butter mixture. Mix until combined.

Step 5

Divide the dough in 3 pieces. Pat them into a 4-by-6-inch rectangle on parchment paper or saran wrap. Wrap dough and refrigerate for 30 minutes.

Step 6

Chop the chocolate into shards.

Step 7

Place one of the dough pieces on a lightly floured surface. Place half of the chocolate on one of the dough pieces. Top with the second dough rectangle. Add remaining chocolate. Top with remaining piece.

Step 8

Roll out (if chocolate falls out, just stick it back in the dough. Lightly flour the top of dough. Roll it out to just about 1/4 inch thickness.

Step 9

Cut out in 2 inch rounds and place on the cookie sheets 2 inches apart. Gather the dough back together and roll back out and cut the rest of the cookies. Refrigerate the cookies again for about 15 minutes.

Step 10

Bake until the cookies are set, 12 to 15 minutes. Let the cookies cool slightly on the baking sheet. Place on a wire rack to cool. Store in an airtight container.