Whisk together the flour, cocoa powder, salt and baking soda in a medium bowl and set aside.
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.
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.
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. (I found mine needed about an hour to soften on the counter before I could roll it out)
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.
Using a round cookie cutter (2-inches in diameter) cut out shapes. For every other cookie, use a small Halloween cookie cutter and cut out the center of each cookie (don't throw these away! Bake these too for a quick snack or top a cupcake with them!) 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.
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.
Meanwhile, make your frosting. In a stand mixer, cream your butter until light and fluffy. Add in your powdered sugar 1 cup at a time with the mixer on low. Add in your heavy cream and vanilla extract and combine. Mix on high for 8 minutes until light and fluffy. Divide into 3 bowls, and add food coloring to 2 of them (I used orange and purple. You could leave it white or just use one color and skip the dividing part). Spread the frosting on the cookie without the center cut out. Top with the cut out center half. Roll the sides in sprinkles if you so choose. Store at room temperature in an airtight container for up to 3 days.
*this recipe was adapted from Food Network