To prepare for rolling, sift the cocoa powder and powdered sugar together onto a shallow plate. Using a teaspoon or small scoop, portion the truffle mixture evenly. Roll quickly between clean palms to form smooth, 1-inch balls, wiping your hands as needed—the mixture will soften with warmth. Roll each truffle in the cocoa mixture, then in the pistachios if using, and transfer to the parchment-lined baking sheet. Chill for about 20 minutes, or until firm. This recipe makes about 20 truffles. Store in an airtight container in the refrigerator for up to 2 weeks, or freeze for up to 3 months.