1. Cream butter and sugars together in a large mixing bowl with a hand mixer or stand mixer on medium speed until light and fluffy, about 2 to 3 minutes.

2. Add the egg, egg yolk, and vanilla extract to the bowl and mix just until smooth and combined, scraping down the sides of the bowl as needed.

3. In a separate bowl whisk together the flour, cocoa powder, baking soda, and salt until evenly combined with no lumps of cocoa.

4. Add the dry ingredients to the wet ingredients and mix on low speed just until a thick, soft dough forms and no streaks of flour remain.

5. Cover the bowl and chill the dough in the refrigerator for at least 30 minutes and up to 24 hours to help the cookies bake up thick and chewy.

6. When ready to bake preheat the oven to 350°F 175°C and line two baking sheets with parchment paper.

7. Scoop about 1.5 tablespoons of dough for each cookie, roll into balls with your hands, and arrange them on the prepared baking sheets leaving about 2 inches between each ball.

8. Bake the cookies for 7 to 8 minutes, until the edges look set but the centers are still soft and slightly puffy.

9. While the cookies bake, cut each large marshmallow in half crosswise so you have 24 pieces ready to go.

10. Remove the trays from the oven and immediately press one marshmallow half, cut side down, into the center of each cookie.

11. Return the cookies to the oven for 2 minutes, just until the marshmallows puff and begin to melt but are not browned.

12. Let the cookies cool on the baking sheets for 5 minutes, then carefully transfer them to a wire rack to cool completely.

13. To make the glaze, combine the chocolate chips and heavy cream in a heat safe bowl and warm in the microwave in 20 second bursts, stirring after each, until the chocolate is melted and smooth.

14. Whisk the powdered sugar, vanilla extract, and pinch of salt into the warm chocolate mixture until you have a thick but pourable glaze.

15. Spoon about 1 tablespoon of glaze over the top of each cooled marshmallow cookie, letting it drip slightly down the sides for a hot cocoa look.

16. Immediately decorate the glazed cookies with holiday sprinkles so they stick before the glaze sets.

17. Allow the cookies to rest at room temperature until the glaze is firm to the touch, then serve or store in an airtight container.