1 stick (8 Tbsp.) unsalted butter, at room temperature
3 Tbsp. creamy peanut butter
2/3 cup packed dark brown sugar
1/4 cup sugar
1/2 tsp. fleur de sel or coarse sea salt
1 tsp. vanilla
3/4 cup mini peanut butter cups, roughly chopped
Active time: 30 minutes
Total time: About 4 hours
Sift the flour, cocoa powder and baking soda together.
Working with a stand mixer with a paddle attachment, beat the butter and peanut butter until smooth and creamy. Add sugars, salt, and vanilla and beat for 2 more minutes.
Turn off the mixer. Pour the dry ingredients in, drape a kitchen towel over the bowl to keep flour from flying, and pulse the mixer on low a few times. Take the towel off and mix on low speed for about 30 seconds, or just until the flour disappears. Don't worry if it looks a little crumbly—that contributes to the sandy texture of these cookies. Toss in the mini peanut butter cups and mix just until incorporated.
Turn the dough out onto a work surface, gather it together, and divide into 2 sections. Working with one half at a time, shape into logs 1 1/2 inches in diameter. Cover the logs in plastic wrap and refrigerate for at least 3 hours. They can be refrigerated up to 3 days or frozen up to 2 months.
When ready to bake, place a rack in the center of the oven and preheat to 325°F. Use a nonstick sheet tray, or line one with parchment paper or a silicone mat. Slice the cookies using a sharp knife into 1/2 inch-thick pieces. If they crack, just squeeze the bits together—they'll bake up just fine. Arrange the rounds on the cookie sheets with at least 1 inch between them.
Bake the cookies 1 sheet at a time for 12 minutes. They won't look done, but will firm up outside of the oven. Transfer the cookies to a cooling rack and let cool until they are just warm, at which point you can serve them. Packed airtight, the cookies can be stored at room temperature for 3 days, or frozen up to 2 months.