Chocolate Whoopie Pies with Peppermint Filling

In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. Set aside. In the bowl of an electric mixer fitted with paddle attachment, beat together sugars, yogurt, oil, eggs, and vanilla on medium speed until mixed, about 3 minutes. Scrape down sides of bowl as needed. Reduce mixer speed to low; gradually add flour mixture and mix for 30 seconds. Cover and refrigerate batter until chilled, about 30 minutes. Preheat oven to 350 degrees F. Transfer batter to a pastry bag fitted with a large plain tip (about 1/2 inch wide). Pipe 2-inch circles onto parchment-lined baking sheets, spacing whoopies 1 1/2 inches apart. (Keep batter refrigerated between batches.) Bake until whoopies spring back when touched, about 9 minutes. Cool completely on pans. To make filling: In the bowl of an electric mixer fitted with paddle attachment, beat together cream cheese and butter on medium speed until fluffy, about 3 minutes. Scrape down sides of bowl as needed. Reduce mixer speed to low; add peppermint extract and salt, and mix for 30 seconds. Gradually add confectioners' sugar and mix for 1 minute. If filling is too soft to hold its shape, add more confectioners' sugar, 1 tablespoon at a time, until desired consistency. (Store in refrigerator, in an airtight container with plastic wrap pressed onto surface of filling, for up to 5 days.) To fill pies: Turn half the whoopies over so they're bottom-side up. Using a pastry bag or butter knife, add a dollop (1 1/2 tablespoons) of Peppermint Filling onto each whoopie bottom. Top with remaining whoopies.