Make the ganache filling by adding the dark chocolate chips to a medium bowl. Heat the heavy cream in a medium saucepan over medium heat until it begins to simmer. Do not let the cream boil over. Remove from heat and pour the heavy cream over the chocolate chips. Let stand for about 1-2 minutes. Stir the heavy cream and dark chocolate chips together until the chocolate melts and the mixture comes together as a creamy frosting. Set aside and allow to rest at room temperature for 1 hour.

Mix flour, cocoa powder, salt, and baking soda in a medium bowl and set aside.

Beat butter in a stand mixer (or with an electronic hand mixer) until creamy. Add both sugars and beat with the butter until combined. Next add the milk, vanilla, egg, and food coloring and beat until combined. Then add the dry ingredients (flour, cocoa, salt and baking soda) to the mixture in small batches and beat until all the dry ingredients are incorporated. Set the dough in the refrigerator and chill for an hour. Chilling the dough really helps the final texture of the cookie, trust me!

Once the dough is chilled, preheat the oven to 350 degrees. Form small balls of dough (about a tablespoon and a half) and add to a lined baking sheet. Bake for 10 to 11 minutes. Remove from oven and allow to cool completely on wire racks.

After the cookies have been cooled, you can make your cookie sandwiches. Add a small dollop of ganache filling to the center of the bottom of a cookie. Top with another cookie and push together so the ganache spread evenly. Repeat this process until all of your sandwiches have been made. Allow cookies to rest for about 30 minutes before eating.