Step 1: Prepare the Cookie Dough

1. Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
2. In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. This should take about 2-3 minutes using a hand mixer or stand mixer.
3. Beat in the egg and vanilla extract, mixing until well combined.
4. In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix.
5. Scoop tablespoon-sized portions of dough and roll them into balls. Place them on the prepared baking sheets, spacing them about 2 inches apart.

Step 2: Bake the Cookies

1. Bake the cookies in the preheated oven for 8-10 minutes. The cookies should be slightly underbaked in the center to ensure a soft, fudgy texture.
2. Remove the cookies from the oven and immediately press a halved marshmallow (cut side down) into the center of each cookie.
3. Return the cookies to the oven and bake for an additional 2 minutes, or until the marshmallows are soft and slightly puffed.
4. Remove the cookies from the oven and let them cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.

Step 3: Make the Ganache

1. While the cookies cool, prepare the ganache. In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer (do not let it boil).
2. Remove the cream from the heat and pour it over the semisweet chocolate chips in a heatproof bowl. Let it sit for 2-3 minutes, then stir until smooth and glossy.
3. Allow the ganache to cool slightly so it thickens but remains pourable.

Step 4: Assemble the Cookies

1. Using a spoon, drizzle the ganache over the marshmallow-topped cookies. Alternatively, you can dip the marshmallow tops directly into the ganache for a smoother finish.
2. Immediately sprinkle festive holiday sprinkles or crushed peppermint candies over the ganache while it’s still wet. This ensures the decorations stick.
3. Let the cookies sit at room temperature until the ganache sets, about 15 minutes.