1. Prepare the Crust:

1. Preheat the oven to 375°F (190°C).
2. Roll out the pie crust and fit it into a 9-inch pie pan. Trim and crimp the edges as desired.
3. Prick the bottom of the crust with a fork to prevent puffing. Line with parchment paper and fill with pie weights or dried beans.
4. Bake for 12–15 minutes until lightly golden. Remove the weights and bake for another 5 minutes. Let the crust cool completely.

2. Make the Chocolate Filling:

5. In a medium saucepan, whisk together the sugar, flour, cocoa powder, and salt.
6. Gradually whisk in the milk until smooth. Cook over medium heat, stirring constantly, until the mixture thickens and comes to a gentle boil, about 7–10 minutes.
7. Remove from heat. Slowly whisk a small amount of the hot chocolate mixture into the beaten egg yolks to temper them, then return the tempered yolks to the saucepan.
8. Cook for 2 more minutes, stirring constantly. Remove from heat and stir in the butter and vanilla extract until smooth.

3. Assemble the Pie:

9. Pour the warm chocolate filling into the cooled pie crust. Smooth the top with a spatula.

4. Add the Topping (Optional):

10. If using whipped cream, chill the pie until completely set (at least 4 hours), then spread or pipe whipped cream on top.
11. For meringue, spread beaten meringue over the warm filling, ensuring it touches the edges of the crust. Bake at 350°F (175°C) for 10–12 minutes until the meringue is golden.