Preheat the oven to 375°F. Grease one 24-count mini muffin tin (or two 12-count mini muffin tins) with non stick spray and set aside.
Whisk together the flour, cocoa powder, baking soda and salt and set aside.
1 cup all-purpose flour, 1/2 cup cocoa powder, 1 teaspoon baking soda, 1/2 teaspoon salt
In a large bowl of a stand mixer fitted with the paddle attachment, or electric mixer, beat the butter and sugars together until light and fluffy, at least 3 minutes. Add the egg, milk and vanilla and beat until combined.
1/2 cup unsalted butter, 1/3 cup granulated sugar, 1/2 cup light brown sugar, 1 large egg, 2 Tablespoons milk, 1 teaspoon vanilla extract
Slowly add the dry ingredients and beat on low speed the turn the speed up to high and beat until combined, scraping down the bottom and sides of the bowl as needed. It will be slightly wet and sticky.
Use a heaping teaspoon to scoop the cookie dough and roll into balls. Place in the prepared muffin tins. Bak for 8-10 minutes or until the cookies puff up and set. Immediately press the back of a teaspoon into the center of the baked cookies when they come out of the oven.
14 jumbo marshmallows
Switch the oven to broil then place half of the large marshmallow cut side down inside the hole of the cookies. Broil the cookies on high for 1-3 minutes, until the marshmallows are puffy and golden brown on the top. Keep an eye on them the whole time because they can burn quickly. Allow them to cool in the pan for 5 minutes then carefully transfer to a wire rack.
While they’re cooling, make the chocolate ganache. Mix all of the ganache ingredients together in a medium bowl. Spoon the ganache over each cookie, partially covering the marshmallow. Top with sprinkles if desired.
1 cup powdered sugar, 2 Tablespoons unsalted butter, 2 Tablespoons cocoa powder, 2 Tablespoons hot water, 1/4 teaspoon vanilla extract