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Place the butter in a large mixing bowl and beat with an electric mixer set to medium speed until the butter is very creamy. Add the sugars and beat on medium for 3 minutes more. Beat in the vanilla and peppermint extracts.
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Add the eggs one at a time, beating well after each addition. Turn mixer to low and beat in the cocoa powder, salt, and baking soda until fully incorporated. With mixer still on low, beat in the flour until just incorporated.
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Use a rubber spatula to fold in the chocolate chips and chopped mints. Loosely cover. dough with plastic wrap and refrigerate for at least 1 hour (or up to 2 days).
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Preheat the oven to 375 degrees. Line two large baking sheets with parchment paper.
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Form the cookie dough into balls of about 2 tablespoons of dough and arrange on baking sheets 2 inches apart. (At this point you can freeze the dough, then transfer the dough balls to freezer bags and freeze for up to 3 months. Bake directly from the freezer, adding another minute or two to the baking time.)
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Bake cookies for 9 to 12 minutes, until set around the edges but still a little puffy in the middle. Let cool on the baking sheets for 4 minutes, then carefully transfer to a wire rack to cool completely. Serve.
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Uneaten cookies can be stored in an airtight container for up to 3 days or in the freezer for up to 3 months.