Prepare pie crust according to package directions and let cool completely.
For the filling: In a saucepan, whisk together the sugar, flour and cocoa powder. Add in the milk, egg yolks, vanilla and butter. Cook over medium heat, whisking frequently, until thickened to a pudding-like consistency, about 5-7 minutes.
Pour the hot filling into the pre-baked pie shell and smooth the top with a spatula.
For the meringue: In a large clean bowl with clean beaters, beat the egg whites with the cream of tartar until stiff peaks form, about 3-5 minutes. Gradually add in the 1/4 cup sugar while beating until thick and glossy, 1-2 minutes more.
Spread or pipe the meringue over the chocolate filling, creating swirly peaks and valleys.
Bake at 350°F for 10-15 minutes until the meringue is lightly golden brown.
Allow pie to cool completely at room temperature, then refrigerate for at least 1 hour before slicing and serving.