Preheat the oven to 350ºF.

In a saucepan, whisk together the sugar, cocoa powder, and flour.

Blend in the evaporated milk and vanilla.

Add the beaten egg yolks and stir into the saucepan.

Add the butter and place the saucepan over low heat, stirring constantly until the butter is completely melted.

Pour the pie filling into the unbaked pie shell. Bake for 35-40 minutes, or until the pie is not wobbly when shaken.

Using a stand mixer on high speed with a whisk attachment, beat the egg whites until soft peaks begin to form. Gradually add the sugar, beating constantly, until stiff peaks begin to form.

Spread the meringue on the pie and return the pie to the oven. Bake until the meringue is golden, about 5 minutes.