1. 1. Preheat your oven to 350°F.

2. 2. Prepare the pie crust by lightly greasing a 9-inch pie dish and pressing the crust into the bottom and up the sides.

3. 3. In a medium saucepan, whisk together the sugar, cocoa powder, flour, and salt. Gradually pour in the milk, stirring constantly to prevent lumps from forming.

4. 4. Place the saucepan over medium heat and cook, stirring frequently, until the mixture thickens and comes to a gentle boil, about 8-10 minutes.

5. 5. Slowly pour a small amount of the hot mixture into the beaten egg yolks, whisking constantly to temper the eggs. Then, pour the egg mixture back into the saucepan, continuing to whisk until well combined.

6. 6. Reduce the heat to low and cook, stirring constantly, until the filling has thickened to a custard-like consistency, about 2-3 minutes.

7. 7. Remove the saucepan from the heat and stir in the butter and vanilla extract until fully incorporated.

8. 8. Pour the warm chocolate filling into the prepared pie crust, smoothing the top with a spatula.

9. 9. Bake the pie for 30-35 minutes, or until the filling is set and the crust is lightly golden.

10. 10. Allow the pie to cool completely on a wire rack before serving, about 2-3 hours.

11. 11. For best results, chill the pie in the refrigerator for at least 4 hours, or up to 24 hours, before serving.