In a large mixing bowl, use an electric hand mixer to beat butter and sugar until light and fluffy. Add eggs and vanilla extract, then beat again to combine.
In another bowl, stir together flour, cocoa, baking soda, and salt. Gradually incorporate this into the butter mixture.
Use a wooden spoon or mixing spatula to stir in peppermint chips. The mixture will be THICK.
Place the bowl in the refrigerator for 10 minutes. Meanwhile, set the oven to 350°F to let it preheat while the cookie dough chills.
Using a medium-sized cookie scoop or regular-sized ice cream scoop, drop dough onto a cold greased cookie sheet, leaving at least an inch of space between the scoops. I used a regular-sized ice cream scoop for extra-large cookies.
Transfer the pan to the preheated oven and bake the cookies for 17 minutes, or until the edges are crispy and the centers are still a little bit gooey.
Remove from the oven and allow cookies to cool on the cookie sheet for 2 minutes. Then, use a thin spatula to transfer them to a cooling rack to cool them completely.