Step 1

to 350°F (175°C). Line a baking sheet with parchment paper.

Step 2

In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Step 3

In a large bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy. Add the egg and vanilla extract, and mix until combined.

Step 4

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Step 5

onto the prepared baking sheet, about 2 tablespoons per cookie, spacing them 2 inches apart.

Step 6

for 10-12 minutes, or until the edges are set.

Step 7

Remove the cookies from the oven and immediately press a marshmallow half into the center of each cookie. Return to the oven and bake for an additional 2 minutes, or until the marshmallows are puffed.

Step 8

Allow the cookies to cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Step 9

with chocolate ganache in a swirl pattern.