Chocolate Grasshopper Ice Cream Tart

Pulse cocoa, 1 Tbsp. malted milk powder (if using), and two-thirds of Oreos in a food processor until coarsely ground with no large pieces remaining. With motor running, gradually add butter; process, scraping sides once, until mixture resembles wet sand (do not overprocess). Press mixture very firmly into bottom and 1 1/2" up sides of springform pan. Freeze until firm, 20-25 minutes.
Meanwhile, peel away sides of containers from ice cream. Cut ice cream into 2" pieces and place in a medium bowl. Let sit until slightly softened, about 5 minutes, then stir with a wooden spoon or large rubber spatula until the consistency of thick cake batter. Break remaining Oreos into large pieces; fold into ice cream along with mints.
Scrape ice cream into crust; smooth top. Freeze until hard, at least 1 day.
Just before serving, place chocolate, corn syrup, salt, and remaining 1 Tbsp. malted milk powder (if using) in a medium bowl. Bring cream to a simmer in a small saucepan; pour over chocolate. Let sit until chocolate is melted, about 5 minutes; whisk until smooth. Pour over tart and spread to edges. Freeze until ganache is firm to the touch, 10-15 minutes. Cut tart into wedges with a hot dry knife to serve (run knife under hot water and wipe dry between slices).
Tart (without ganache) can be made 1 week ahead. Once solid, wrap in plastic; keep frozen.