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

Cream together the softened butter and granulated sugar in a bowl until light and fluffy.

Add the fine sea salt and vanilla extract, mixing until well combined.

Incorporate the egg whites into the creamed mixture, mixing until fully integrated.

Whisk together the all-purpose flour, black cocoa powder, and baking soda in a separate bowl.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Chill the dough in the refrigerator for about 30 minutes if sticky. Roll out the dough to ¼ inch thick and cut into shapes.

Place the cut cookies on the prepared baking sheets and bake for about 10-12 minutes.

Allow the cookies to cool on the sheets for a few minutes, then transfer to a wire rack and sandwich with your favorite filling.