It’s crucial that this tart be frozen solid before slicing. If you are in and out of your freezer a lot, that may take longer than one day; err on the safe side.
3 tablespoons unsweetened cocoa powder
2 tablespoons malted milk powder, divided (optional)
1 14.3-ounce package Oreos, divided
3 tablespoons unsalted butter, melted
2 pints green mint chip ice cream
15 Andes crème de menthe thins or 8 After Eight mint chocolate thins
4 ounces bittersweet or semisweet chocolate, chopped
1 tablespoon light corn syrup
Pinch of kosher salt
1/2 cup heavy cream
A 9-inch springform pan
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).
Do Ahead: Tart (without ganache) can be made 1 week ahead. Once solid, wrap in plastic; keep frozen.
Recipe by Claire Saffitz