Make the crust: Whisk together flour and salt. Cut in cold butter until mixture resembles coarse crumbs. Gradually add ice water, mixing until dough comes together. Form into a disc, wrap, and chill for at least 30 minutes.

Roll out dough into a 12-inch circle. Transfer to a 9-inch pie plate. Trim and crimp edges. Prick bottom with a fork.

Line crust with parchment paper and fill with pie weights. Bake at 375°F (190°C) for 15 minutes.

Remove parchment paper and weights. Bake for another 5-7 minutes, or until lightly golden.

Make the filling: Whisk together sugar, cocoa powder, flour, and salt. Add eggs one at a time, whisking well. Gradually whisk in milk until smooth.

Stir in vanilla and melted butter (if using). Pour filling into baked pie crust.

Bake at 350°F (175°C) for 35-40 minutes, or until filling is set (center may jiggle slightly).

Let cool completely on a wire rack before serving. Refrigerate for best results.