Instructions

 Adjust oven rack to middle position and heat oven to 325 degrees. Grease 8-inch square baking pan, line with parchment paper, grease parchment, and flour pan.
 Whisk sugar, flour, baking soda, and salt together in medium bowl; set aside. Combine ½ cup milk, chocolate, and cocoa in medium saucepan. Place saucepan over low heat and cook, whisking frequently, until chocolate is melted and mixture is smooth. Remove from heat and let cool slightly, about 5 minutes. Whisk oil, eggs, and vanilla into chocolate mixture (mixture may initially look curdled) until smooth and homogeneous. Add sugar mixture and whisk until combined, making sure to scrape corners of saucepan.
 Scrape batter into prepared pan and smooth top with rubber spatula. Gently tap pan on counter to release air bubbles. Bake until toothpick inserted in center comes out clean, 30 to 35 minutes, rotating pan halfway through baking. Let cake cool in pan on wire rack for 10 minutes. Remove cake from pan, discarding parchment, and let cool completely on wire rack, about 2 hours. (Cake can be wrapped in plastic wrap and stored at room temperature for up to 24 hours.)
 In bowl of stand mixer fitted with paddle, break cake into rough 1-inch pieces. Add remaining 2 tablespoons milk and beat on medium-low speed until broken into fine, evenly moistened crumbs and cohesive dough begins to form, about 1 minute, scraping down bowl as needed.
 Line baking sheet with parchment paper. Working with 1 packed tablespoon cake mixture at a time, roll into balls and place in single layer on prepared sheet. Cover with plastic wrap and freeze until firm but still pliable, 45 minutes to 1 hour.
 Place cookies in zipper-lock bag and crush with rolling pin until broken into small pieces; transfer to small bowl. Microwave 2 cups chocolate chips in 2-cup liquid measuring cup at 50 percent power, stirring occasionally, until melted, 1 to 2 minutes. Working with 1 cake ball at a time, insert lollipop stick into cake ball, stopping at center. Dip entire cake ball into melted chocolate and turn until completely coated. Lift up from chocolate (do not turn upright) and gently shake side to side to allow excess coating to drip off evenly. Turn cake pop upright and gently twist back and forth to even out coating. Dip into crushed cookies, pressing gently to adhere. Turn cake pop upright and insert stick into cake pop stand. Stir and rewarm chocolate in microwave as needed to stay fluid; add remaining ½ cup chocolate chips and melt when chocolate level becomes too low to dip easily. (If cake balls become too soft, refreeze until firm.)
 Let cake pops stand at room temperature until coating is set, about 30 minutes. Serve. (Assembled cake pops can be kept at room temperature for up to 4 hours or refrigerated for up to 3 days; bring to room temperature before serving.)