Start by placing your unbaked pie crust into your 9-inch pie dish, pressing it gently into the edges. Crimp the border however you like, whether that's a simple fork press or a fancier fluted edge. Keep the crust chilled in the fridge while you make the filling so it stays firm.
In your large bowl, whisk the sugar, cornmeal, flour, and salt together first so there are no lumps. Then add your melted butter, eggs, evaporated milk, vanilla, and vinegar, whisking until everything looks smooth and well combined. This usually takes me about two minutes of steady whisking.
Carefully pour your filling into the chilled pie crust, leaving a little room near the top since the custard will puff slightly while baking. I like to give the dish a gentle shake to settle the mixture evenly before it goes into the oven.
Bake your pie at 350°F for about 45 to 50 minutes, until the top turns golden brown and only the very center has a slight jiggle. If the edges of your crust start browning too fast, loosely cover them with foil halfway through baking.
Once baked, move your pie to a cooling rack and let it sit at room temperature for at least two hours. This cooling step is what lets the custard fully set, so resist the urge to cut into it early no matter how good your kitchen smells.