In a medium saucepan over a double-boiler on medium heat, stir the chocolate and butter together with a wooden spoon until melted. Add the sugar and espresso powder and continue to stir until combined. Remove from the heat and add the eggs, one at a time, followed by the vanilla extract.

Sift the cocoa powder, cornstarch and salt into the batter. Stir until the batter is smooth and glossy.

Fold in the 1/2 cup of M&Ms. Pour batter into the prepared baking dish and bake for 23-25 minutes or until the top and edges are set and the center isn't jiggly. Remove from oven and allow to cool completely.

Once brownies have cooled, prepare the chocolate ganache. In a small microwave-safe bowl combine the chocolate and heavy cream. Microwave for 1 minute, remove, and cover with plastic wrap. Let it sit for 5 minutes. Remove plastic wrap and stir until smooth and glossy. Set ganache aside to allow it to cool and thicken.

Make the buttercream. Using a stand mixer fitted with a paddle attachment, whip the butter until smooth and creamy. Add the powdered sugar and continue whipping, stopping and scraping down sides of bowl to allow everything to blend more evenly. Mixture will appear a little dry.

Add the vanilla, mint extract, food coloring, and heavy cream (or milk), and continue whipping the buttercream until light and fluffy, about 1 minute. Spread evenly onto the cooled brownies.

Pour the ganache over the buttercream, tilting the dish slightly to evenly distribute the ganache. Sprinkle with the chopped 1/2 cup of M&Ms. Refrigerate for 15 minutes or until ganache has set. Cut into squares and serve.