In a large bowl, cream the softened butter and granulated sugar together using an electric mixer on medium speed until light and fluffy, about 3-4 minutes. Scrape down the sides of the bowl as needed.

Add the egg yolk, vanilla extract, and fine sea salt. Mix on low speed until just combined.

In a separate bowl, whisk together the all-purpose flour and cocoa powder to ensure even distribution.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just incorporated. Be careful not to overmix. The dough will be crumbly.

Turn the dough out onto a clean work surface. Use your hands to bring it together into a cohesive ball.

Place a large sheet of parchment paper on your work surface. Place the dough onto the parchment paper.

Using the parchment paper as a guide, shape the dough into a tight, uniform cylinder, approximately 2 inches in diameter. Ensure there are no air pockets. Press firmly and evenly.

Once the cylinder is formed, wrap it tightly in the parchment paper, twisting the ends like a candy wrapper to maintain its shape and prevent it from expanding during chilling.

Refrigerate the dough cylinder for at least 2 hours, or until very firm. This is crucial for clean slicing.

Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.

Remove the chilled dough cylinder from the refrigerator. Unwrap it from the parchment paper.

Using a sharp chef's knife, carefully slice the dough cylinder into uniform rounds, about 1/4-inch thick. Aim for geometric precision.

Arrange the cookie slices on the prepared baking sheets, spacing them about 1 inch apart.

Sprinkle the tops of the cookies lightly with flaky sea salt.

Bake for 12-15 minutes, or until the edges are set and the centers are still slightly soft. The cocoa powder can make it difficult to judge browning, so watch the edges closely.

Allow the cookies to cool on the baking sheets for 5 minutes before carefully transferring them to a wire rack to cool completely.