Preheat oven to 350°F. Place the unbaked pie shell on a baking sheet.
Melt the butter and let it cool slightly so it doesn’t scramble the eggs.
Whisk sugar, cocoa powder, flour, and salt in a large bowl until evenly combined.
Stream in the melted butter, whisking until glossy and thick.
Whisk in the eggs one at a time until smooth and well blended.
Stir in the evaporated milk and vanilla until the batter looks silky.
Fold in pecans and chocolate chips if using.
Pour the filling into the pie shell and smooth the top.
Bake 32–38 minutes, until the edges are set and the center has a soft jiggle.
Cool on a rack for at least 1 hour to set before slicing.