Sift and mix flour, cocoa powder, and salt in a medium sized bowl and set aside.
Place butter and sugar in a mixing bowl of a stand mixer fixed with the paddle attachment. Mix on medium speed for 1-2 minutes until pale and fluffy. Use a spatula to scrape any ingredients from the side of the bowl.
Add egg yolk and beat for 30 minutes until combined.
Add dry ingredients in two additions and mix on low speed until cookie dough forms. Do not over mix.
Divide cookie dough in 2 parts, flatten to a disc-shape, and wrap in cling film or plastic wrap. Chill for 1-2 hours up to overnight.
Once cookie dough has chilled, preheat oven to 170°C (340F) and line a large baking sheet with parchment/baking paper.
Allow cookie dough to thaw for 5-10 minutes. Roll out on a lightly floured surface to ⅛ to ¼ inch thickness. Cut out dough using a round or scalloped cookie cutter. Transfer cookie dough to baking sheet, leaving a 1 inch space between each one.
Chill cut-out dough for 15-20 minutes. Bake in a preheated oven for 8-12 minutes until just set. Baking time depends on cookie thickness. Cookies will be soft when hot and will harden as they cool. Transfer to a cooling rack to cool completely before filling.