Preheat the oven to 350°F. Line a 9×13-inch baking pan with parchment paper, leaving an overhang for easy removal.
In a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, and brown sugar until light and fluffy, about 3–4 minutes.
Add the eggs one at a time, mixing well after each, then mix in the peppermint extract.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients on low speed until just combined.
Stir in 4 ounces of the dark chocolate (chopped). Spread the batter evenly into the prepared pan.
Bake 20–25 minutes, until set in the center. Let cool completely in the pan, then lift out using the parchment and slice into squares.
Melt the remaining 4 ounces of dark chocolate in the microwave in 20-second bursts, stirring until smooth. Dip half of each bar into the chocolate, then sprinkle with crushed peppermints.
Place on parchment paper to set until the chocolate hardens. Store in an airtight container for up to a week.
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