Preheat the oven: Position an oven rack in the lower third of the oven. Preheat the oven to 350°F.
Prep the crust: Fit the pie crust into a 9-inch pie plate. Crimp the edges and gently press the crust against the rim so it doesn't slide down in the oven. Refrigerate the crust while you make the filling.
Mix the dry ingredients: In a large bowl, sift or whisk together the sugar, cocoa powder, cornmeal, salt, and espresso powder (if using). Break up any lumps in the cocoa.
Mix the wet ingredients: In a separate bowl, whisk the eggs just until combined. Whisk in the cooled melted butter, corn syrup, vanilla, and vinegar.
Combine: Gradually whisk the wet ingredients into the dry ingredients until the filling is smooth and well combined. Avoid beating too hard, which adds air to the filling.
Fill and bake: Place the crust on a rimmed baking sheet and pour in the filling. Bake for 45 to 50 minutes, until the filling is puffed and the center is just set with a slight jiggle. Check the crust after 30 to 35 minutes, and cover the edges with foil if they're browning too quickly.
Cool: Transfer the pie to a wire rack and cool at room temperature for at least 3 to 4 hours. The filling will continue to set as it cools.
Chill (optional): For the firmest filling and cleanest slices, refrigerate the cooled pie for at least 2 hours or overnight.
Serve: Let a chilled pie sit on the counter for 20 to 30 minutes. Serve with whipped cream or a scoop of vanilla ice cream.