Preheat your oven to 350°F. Line baking sheets with parchment paper
In a large mixing bowl, beat the butter, granulated sugar, and brown sugar together until light and fluffy (about 2 minutes).
Mix in the egg, egg yolk, peppermint extract, and vanilla extract. Beat until fully combined and smooth.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, salt, cornstarch, and espresso powder.
Gradually add the dry ingredients to the wet ingredients. Mix just until a dough forms—don’t over mix! Mix in the dark chocolate chips.
Using a medium cookie scoop (about 60 grams per cookie), scoop out heaping balls of dough. Roll them between your hands to create smooth, uniform balls. Place them on the prepared baking sheets, spacing them 2 inches apart.
Position it on the center rack of the oven to ensure even baking, avoiding placement too close to the top or bottom. Bake the cookies for 10 minutes. They will spread slightly, and the edges should look set while the centers remain looking underbaked.
Remove the cookies from the oven and immediately place 2 halves of one marshmallow on top of each cookie. Return to the oven for 1 minute, just until the marshmallows soften.
Let the cookies cool on the baking sheet for 5 minutes. During this time, gently press down on the melted marshmallows with the back of a spoon to spread them slightly.
To scoot a cookie into a perfect circle, grab a round object like a bowl or large cookie cutter slightly larger than your cookie. While the cookies are still warm and soft on the baking sheet, place the bowl or cutter over a cookie and gently move it in circular motions to reshape the edges. This quick trick smooths out uneven edges, giving you perfectly round cookies every time!
Drizzle the cooled down cookies with melted chocolate evenly, then sprinkle with crushed peppermint candies. Let the chocolate set a bit before serving or storing.
See storage in notes below.