1) Prep the pan
Line the bottom of a 9-inch springform pan with parchment paper. Lightly grease the sides. This makes releasing the pie way less stressful.
Using a glass or ceramic pie plate? Skip the freezer steps before baking (see Step 2) to avoid thermal shock. Fridge chilling works just fine.
2) Make the cookie crust
Pulse the cookies in a food processor until fine crumbs. Mix with melted butter and a pinch of salt until it looks like wet sand.
Press firmly into the bottom of the pan (and slightly up the sides if you want).
Set the crust: Freeze for 15 minutes if you are using a metal pan. If you are using a glass or ceramic pie plate, chill the crust in the fridge for 30 minutes instead.
Optional (extra crisp crust): Bake at 350°F for 8 to 10 minutes, then cool completely. This is a good move if you want sturdier slices.
3) Bake the flourless chocolate layer
Preheat oven to 350°F.
In a microwave-safe bowl (or over a double boiler), melt the 6 ounces chocolate with the 1/2 cup butter until smooth. Let it cool until it is warm, not hot (you should be able to hold your finger to the bowl comfortably). This keeps the eggs from scrambling.
Whisk in sugar, then eggs one at a time. Stir in vanilla, salt, and cocoa powder until glossy and smooth.
Pour over the crust and bake for 14 to 18 minutes, until the edges look set, a thin crust forms on top, and the center is still a little soft and barely jiggly. Cool completely at room temperature, then freeze for 30 minutes (metal pan), or refrigerate until well chilled (glass or ceramic).
4) Make the coffee fudge sauce
Warm the heavy cream and hot coffee until steaming (microwave is fine). Add chocolate, brown sugar, butter, and salt. Let sit 1 minute, then whisk until smooth and shiny.
Cool the sauce to room temperature, until thickened to a spreadable but still pourable consistency. If it is warm, it will melt your ice cream and make a mess.
Spread a thin layer (about 1/3 to 1/2 cup) over the chilled chocolate base. Freeze 15 minutes so it firms up (or refrigerate if using glass or ceramic). Extra sauce? Save it for drizzling on slices.
5) Add the ice cream layer
Scoop softened vanilla ice cream onto the pie and spread into an even layer. Press it in so there are no air pockets. Freeze until solid, at least 4 hours (overnight is best).
6) Whip the cream and finish
Right before serving, whip 1 cup cold heavy cream with powdered sugar and vanilla to soft peaks (or a little firmer if you like it more stable). Spread on top and finish with chocolate shavings, mini chips, or crushed cookies.
Want to decorate ahead? Whipped cream is best day-of. For a more stable topping, whip in 2 tablespoons mascarpone or cream cheese.
7) Slice and serve
Release the springform ring. For clean slices, use a hot, dry knife (run it under hot water, wipe dry, slice, repeat). Serve immediately and return leftovers to the freezer fast.