Combine Dry Ingredients: In a medium bowl, whisk together the flour, cocoa powder, cinnamon, and salt. Set aside.

Cream Butter and Sugar: In a large bowl, use an electric mixer to cream the softened butter and granulated sugar together on medium-high speed until light and fluffy, about 2-3 minutes.

Add Wet Ingredients: Beat in the egg and vanilla extract until just combined, scraping down the sides of the bowl as needed.

Incorporate Dry Ingredients: Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until the dough just comes together and no dry streaks remain.

Chill Dough: Divide the dough in half. Shape each half into a flat disk, wrap tightly in plastic wrap, and refrigerate for at least 1 hour (and up to 2 days).

Preheat Oven: Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

Roll and Cut: On a lightly floured surface, roll out one disk of dough to about ¼-inch thickness. Use bone-shaped or other Halloween cookie cutters to cut out shapes. Transfer cut-outs to the prepared baking sheets, spacing them about 1 inch apart.

Bake: Bake for 8-10 minutes, or until the edges are set. The cookies will still be soft but will firm up as they cool.

Cool: Allow cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Decorate: Prepare royal icing by beating powdered sugar, meringue powder, and water until stiff peaks form. Pipe onto cooled cookies as desired.