Preheat oven to 375ºF and line baking sheets with parchment paper. 

Using a hand mixer or stand mixer fitted with a paddle attachment, cream the butter, sugar, molasses, cocoa powder, and egg yolk until well combined. Scrape down the bowl to ensure everything is evenly incorporated.

Sift the dry ingredients into a separate bowl. Slowly add them to the wet ingredients, mixing just until a soft dough forms with the mixer on low.

Turn the dough out onto a lightly floured surface and knead it until it is smooth and all ingredients are well combined. Divide the dough in half, shape each half into a disc, and wrap them in plastic wrap. Chill the dough in the refrigerator for 20 minutes.

Lightly flour a workspace and roll out one disc at a time to ¼ inch thickness with a rolling pin.

Dust the dough with granulated sugar to prevent the sticking of cookie stamps and cutters.

Press the stamp into the dough using your favorite cookie cutters or stamps. If you're using a plunger-style stamp, stamp the cookie first and then use the cutter to create a clean cut.

Transfer the cutout cookies to the parchment-lined baking sheet.

Chill the cut-out cookies for 10 minutes. 

Repeat the process with the remaining dough scraps, chilling the dough if it is sticky.

Bake one cookie sheet at a time for 8 minutes for soft, chewy cookies and 9-10 minutes for firm cookies.

Allow the cookies to rest for 10 minutes before transferring them to a wire rack to cool completely before glazing.

To make the glaze, combine the powdered sugar, vanilla extract, and a little milk at a time in a medium bowl until smooth. The icing should be thick.

Use a pastry brush to coat the tops of the cooled cookies with icing. Allow the icing to dry completely before stacking or storing them in an airtight container.