Preheat the oven to 375 degrees F., and grease a 9-inch deep dish or 10-inch standard pie plate, making sure to grease all the way up the sides. (I once made this recipe in a standard 9-inch pie plate. It worked but was very)
When separating the eggs, place the yolks in a large mixing bowl and the whites in the bowl of a stand mixer (or another mixing bowl if using a handheld mixer). Whisk the vanilla into the egg YOLKS, and set aside.
Melt the chocolate and butter in a heavy saucepan over low heat, stirring until smooth.
Gradually stir the melted chocolate mixture into the yolk mixture (I do it in 3 to 4 globs), and whisk until blended. This mixture will be fairly thick.
Beat the egg whites at low speed until foamy. Slowly increase the speed to medium-high to high and beat until soft peaks form. At this point, gradually add the sugar, and beat until stiffer peaks form. (Tip: When in doubt, err on the side of softer peaks to avoid overbeating. If your peaks look fluffy and hold their shape, they’re good.) Gradually fold the beaten egg whites into the chocolate mixture until the ingredients are just blended. (Again, I do it in 3 to 4 additions.) Pour the mixture into the prepared pie plate.
Bake for 25-35 minutes, depending on the chosen pie dish, until the pie is just cooked through the center, being careful not to over bake. (Like brownies, you may prefer the slightest hint of an under-baked center, but it shouldn’t be jiggly.) Drape with a piece of aluminum foil if the edges are turning dark before the pie is done. The pie may crack on the top and will sink some when cooling; this is normal.
Let the pie cool. Store, covered, at room temperature for 5 to 7 days. For a pretty look, dust with powdered sugar prior to serving, if desired. Serve with whipped cream or ice cream.
