Heat the oven to 350°F. Fit the unbaked pie crust into a 9-inch pie plate and chill it while the filling is mixed.
Whisk the sweetened condensed milk, evaporated milk, cocoa powder, light brown sugar, melted butter, cornstarch, cornmeal, vanilla extract, apple cider vinegar, and fine salt until completely smooth.
Pour the filling into the chilled crust and smooth the surface. Place the pie on a baking sheet to catch any drips.
Bake for 42 to 48 minutes until the outer 2 inches are set and the center still has a gentle wobble.
Cool the pie completely at room temperature. The filling will continue to firm as it cools.
Refrigerate for at least 3 hours before serving for clean slices and a dense, fudgy custard texture.
