Line the bottom and sides of an 8″ springform pan with plastic wrap, leaving enough overhang to cover the top.
Make ice cream pie: In the bowl of a food processor, pulse Oreos until fine crumbs form. Add butter and pulse until combined and the mixture resembles wet sand.
Pour cookie crumbs into springform pan and, using your hands or the bottom of a small measuring cup, press into an even layer. (If cookies are sticky, grease your hands or the measuring cup with nonstick cooking spray.) Top with softened ice cream, spreading into an even layer. Cover tightly with plastic wrap overhang and set in freezer for at least 6 hours until very firm.
Before serving, make fudge sauce: In a small heavy-bottomed saucepan over low heat, melt chocolate and butter. Stir in sugar, milk, and cornstarch and cook, stirring often, until sauce begins to thicken and sugar has dissolved. Remove from heat; stir in vanilla and salt.
Remove pie from pan (for softer ice cream, let sit up to 30 minutes), slice into wedges, and top with fudge sauce. Freeze leftovers wrapped tightly in plastic for up to 2 weeks. Refrigerate sauce in airtight jar for up to 1 week.
