Prepare the Crust: If using a store-bought pie crust, follow the package instructions to bake it (usually blind-bake it at 350°F for about 10–12 minutes). If making homemade, bake the pie crust and let it cool completely before filling.

Make the Chocolate Filling: In a medium saucepan, whisk together sugar, cocoa powder, flour, and salt. Gradually add milk, whisking until smooth. Cook the mixture over medium heat, stirring constantly, until it starts to bubble and thickens (about 8–10 minutes).

Temper the Egg Yolks: In a small bowl, whisk the egg yolks. Slowly pour a little of the hot chocolate mixture into the egg yolks, whisking constantly to temper the eggs (to avoid curdling). Once tempered, slowly whisk the egg yolks back into the saucepan with the rest of the chocolate mixture.

Cook the Filling: Continue to cook the filling for another 2–3 minutes, stirring constantly, until it becomes thick and smooth. Remove from heat and stir in butter and vanilla extract until fully combined.

Fill the Pie Crust: Pour the chocolate filling into the prepared pie crust. Smooth the top with a spatula and refrigerate for at least 4 hours or overnight to set.

Make the Whipped Cream Topping: In a mixing bowl, beat heavy cream, powdered sugar, and vanilla extract until soft peaks form. Spread or pipe the whipped cream on top of the chilled pie.

Serve: Garnish with chocolate shavings or curls if desired. Slice and enjoy!