Heat the oven to 350°F (175°C). Combine the finely crushed chocolate sandwich cookies, melted unsalted butter, and 1/8 teaspoon salt, then press the mixture firmly over the bottom and sides of a 9-inch pie plate.

Bake the crust for 8 to 10 minutes, then cool it completely. For a no-bake option, refrigerate the pressed crust for at least 30 minutes, though baking gives it better structure.

In a medium saucepan, whisk together the granulated sugar, cocoa powder, cornstarch, and 1/4 teaspoon salt until evenly combined.

Gradually whisk in the whole milk and egg yolks until smooth, then whisk in the instant espresso powder. Make sure no dry pockets remain before placing the saucepan over the heat.

Cook the custard over medium heat, whisking constantly, until it thickens and begins to bubble. Continue cooking for about 1 minute after bubbling starts, watching closely because the bottom can scorch quickly.

Remove the saucepan from the heat and immediately add the chopped semisweet chocolate, 2 tablespoons butter, and vanilla extract. Whisk until everything melts into a smooth, glossy filling.

Pour the warm mocha filling into the cooled crust and press plastic wrap directly against its surface. Refrigerate the pie for at least 4 hours, preferably overnight, until fully set.

Beat the cold heavy cream, confectioners’ sugar, and vanilla until medium-stiff peaks form. Spread or pipe the whipped cream over the chilled pie, then finish with chocolate curls, grated chocolate, cocoa powder, or a small pinch of espresso powder.