Preheat oven to 350℉. Line baking sheets with parchment paper, and set aside.
Melt butter in a medium saucepan over medium heat. When butter is completely melted, add chocolate chips, and stir constantly until chocolate melts completely.
Remove saucepan from heat and stir in the milk, vanilla, and cocoa powder.
Using a stand mixer fitted with a paddle attachment or an electric hand mixer and large mixing bowl, beat together sugar and eggs for 1-2 minutes, until well combined.
Pour in the melted chocolate mixture and beat on medium speed for 1-2 minutes, until well combined.
In a medium sized mixing bowl, whisk together flour, baking soda, and salt (if using). Add half of this to the chocolate mixture; beat on low speed until just incorporated, then add in the second half and mix, until just barely combined.
Add in chopped Andes mints to the bowl of dough. Use a wooden spoon or mixing spatula to gently fold them in.
Use a medium cookie scoop to portion 2-tablespoon sized scoops of dough onto the prepared cookie sheet. Arrange them in a single layer, at least 2 inches apart to allow room for the dough to spread. For extra mint flavor and visual appeal, break up additional Andes mints and press two or three pieces into each dough ball.
Bake in preheated oven for 7-8 minutes. Allow cookies to sit for 5 minutes on the baking sheet before moving them to a wire cooling rack.
Notes
Avoid over mixing the dough, which can cause the cookies to be dry and crumbly.
Store cookies in an airtight container at room temperature for up to a week, or freeze in a freezer-safe container for up to 4 months.
The unbaked dough also freezes well for up to 4 months. If you're baking Andes mint cookies using frozen dough, increase baking time by 5 minutes.
For extra visual appeal and chocolate mint flavor, insert a couple of extra Andes mints into each cookie dough ball before baking.