Preheat your oven to 350°F (175°C). In a large mixing bowl, whisk together the cocoa powder, sugar, and cornstarch until fully blended. Add the beaten eggs, melted butter, evaporated milk, and vanilla extract to the bowl, stirring until the mixture is smooth and cohesive.
Carefully pour the chocolate filling into the unbaked pie shell, avoiding over-filling. Place the pie on the center rack of your oven and bake for 45 to 55 minutes, or until a firm crust has formed over the top and the filling is mostly set. The pie may still have a slight jiggle, which will firm up as it cools.
Allow the pie to cool completely at room temperature before slicing, as this helps it achieve the perfect texture. Serve with a dollop of whipped cream, if desired, for an extra layer of indulgence.