In a large mixing bowl, use a hand mixer to combine the brown sugar, granulated sugar, and softened butter. Mix until light and creamy.

Add the egg, egg yolk, and vanilla extract. Mix until fully incorporated and smooth.

In a separate medium bowl, whisk together the flour, cocoa powder, baking powder, salt, and baking soda.

Gradually add the dry ingredients into the wet ingredients, mixing on low speed until just combined. Be careful not to overmix.

Divide the dough into three equal portions. Wrap each portion tightly in plastic wrap and refrigerate for 30 minutes.

Once the dough has chilled, preheat oven to 350°F. Line a baking sheet with parchment paper. Set aside.

After chilling, place a large sheet of parchment paper on your work surface. Unwrap one portion of dough and roll it out into an 8-inch square about ¼ inch thick. Sprinkle ⅓ of the chocolate chunks evenly over the surface, gently pressing them in.

Unwrap the second portion of dough and roll it out separately into a square about the same thickness. Carefully lay it on top of the chocolate-covered first layer. Press down gently.

Sprinkle ⅓ of the chocolate chunks over the second layer, pressing them in slightly.

Roll out the final portion of dough and place it on top to form a three-layer dough stack. Tuck any loose chocolate chunks back into the dough as needed.

Gently roll the entire stack with a rolling pin until it forms a 10-inch square about ½ inch thick.

Top with the remaining chocolate chunks.

Use a sharp knife to cut the dough into 2-inch squares (yields 25 cookies). Place the squares on the lined baking sheet, spacing them about 1 inch apart.

Bake for 11–14 minutes, or until the edges are set and the centers no longer appear wet.

Let the cookies cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.