2 cups assorted mix-ins (such as holiday M&Ms, toffee bits, shredded coconut, peanut butter chips and/or white chocolate chunks)
Position racks in the upper and lower thirds of the oven and preheat to 375 degrees F. Line 2 baking sheets with parchment paper. Sift the flour, baking soda and salt into a large bowl. In a separate large bowl, whisk the melted butter, brown sugar, cocoa powder, eggs and vanilla until combined. Stir in the flour mixture with a wooden spoon until just incorporated, then stir in the chocolate chips and assorted mix-ins.
Drop tablespoonfuls of dough about 3 inches apart on the prepared baking sheets; smooth the tops with damp fingers. Bake, switching the pans halfway through, until puffed and set, about 12 minutes. Let cool 5 minutes on the baking sheets, then transfer to racks to cool completely.