Step 1

Whisk together the flour, cocoa powder, salt and baking soda in a medium bowl and set aside.

Step 2

Put the butter into the bowl of a stand mixer fitted with a paddle attachment (or a large bowl if using a hand mixer) and beat on medium-high speed until smooth. Add the sugar and beat until light and fluffy, scraping down the sides of the bowl and paddle as needed, about 4 minutes more. Add the egg and vanilla and beat until smooth. Add the melted chocolate and beat until smooth.

Step 3

Reduce the mixer speed to medium-low and add the flour mixture in two batches, scraping down the sides of the bowl and paddle as needed. Mix until incorporated.

Step 4

Divide the dough in half and shape into two flattened disks. Wrap in plastic wrap and refrigerate until firm at least 1 hour and up to 3 days. Remove the dough from the refrigerator for 20 to 30 minutes before rolling, to soften.

Step 5

Position oven racks to the upper and lower third of the oven and preheat to 350 degrees F. Line two baking sheets with parchment. Now flour parchment for rolling: Dust a piece of parchment with flour, place a disk of dough on it, dust the disk with flour, then cover it with a second piece of parchment. Roll out the dough about 1/4 inch thick. Cut out shapes with 11/2- to 2-inch cookie cutters. Gather up the scraps, re-roll and cut out. (Chill the scraps if the dough becomes too soft.) Roll and cut out the remaining disk of dough. Working in batches, arrange the cookies 1 inch apart on the prepared baking sheets.

Step 6

Bake the cookies until the tops look dry and no indentation is left when pressed with your finger, 10 to 12 minutes. Let the cookies cool for a few minutes on the baking sheets, then use a metal spatula to transfer them to a rack to cool completely.

Step 7

Decorate with royal icing as desired. Store at room temperature in an airtight container for up to 3 days.

Step 8

Combine the confectioners' sugar, meringue powder and 3/4 cup water in the bowl of a stand mixer fitted with the whisk attachment (or in a large bowl if using a hand mixer). Mix on medium-low speed until pure white and stiff enough to form peaks.

Step 9

Add up to 1 tablespoon food coloring if using and mix with a rubber spatula until the color is uniform. Store at room temperature, covered, with plastic wrap on the surface for up to 2 days. Re-whip until smooth before using.