1. Preheat your oven to 350°F (175°C) and prepare your pie crust in a pie pan. Bake the crust for 8-10 minutes until it’s lightly golden. Set aside to cool.
2. In a medium saucepan, whisk together sugar, cocoa powder, flour, and salt. Gradually stir in milk until the mixture is smooth.
3. Place the saucepan over medium heat and cook, constantly stirring, until the mixture begins to thicken and boils. This should take about 7-10 minutes.
4. Reduce the heat to low, and slowly pour a small amount of the hot mixture into the beaten egg yolks while whisking continuously. This tempers the yolks, preventing them from scrambling.
5. Return the yolk mixture to the saucepan and cook for an additional 2 minutes, stirring constantly until the filling is thick and creamy.
6. Remove from heat and stir in butter and vanilla until fully incorporated.
7. Pour the chocolate filling into the prepared pie crust, smoothing the top with a spatula.
8. Allow the pie to cool at room temperature for about an hour, then refrigerate until fully set, approximately 4 hours or overnight.
9. Serve chilled, topped with whipped cream if desired.

Remember, the key here is patience. Just like my grandmother taught me, good things come to those who wait, especially when it involves chocolate!