Step 1: Preheat Your Oven. Begin by preheating your oven to 350°F (175°C). Line your baking sheet with parchment paper to prevent the cookies from sticking.
Step 2: Mix Dry Ingredients. In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt. Set this mixture aside.
Step 3: Prepare the Butter and Sugars. In a large mixing bowl, combine the melted and cooled butter with both sugars. Mix until well combined, creating a creamy texture.
Step 4: Add the Eggs and Vanilla. Add the egg, egg yolk, and vanilla extract to the butter-sugar mixture, mixing until fully incorporated.
Step 5: Combine Dry and Wet Ingredients. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
Step 6: Fold in Chocolate Chips. Gently fold in the dark chocolate chips, ensuring they are evenly distributed throughout the cookie dough.
Step 7: Shape and Bake. Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart. Bake in the preheated oven for 10-12 minutes, or until the edges are set.
Step 8: Prepare the Mint Filling. While the cookies are baking, prepare the mint filling. In a medium bowl, beat together the powdered sugar, softened butter, evaporated milk, vegetable oil, vanilla extract, and peppermint extract until smooth and creamy.
Step 9: Assemble the Cookies. Once the cookies are completely cool, spread a spoonful of the mint filling on the bottom of half the cookies and sandwich them with the remaining cookies.
Step 10: Make the Ganache. For the ganache, heat the heavy cream in a small saucepan over low heat until just simmering. Pour it over the chopped dark chocolate in a bowl and let it sit for a few minutes. Stir until smooth and glossy.
Step 11: Dip the Cookies. Dip the filled cookies into the ganache, allowing any excess to drip off. Place them back on the parchment-lined baking sheet and let the ganache set before serving.
