Step 1: Prepare the Pie Crust

1. Mix Dry Ingredients: In a large bowl, whisk together flour and salt.
2. Cut in Butter: Add cold butter cubes and use a pastry cutter or your fingers to blend until the mixture resembles coarse crumbs.
3. Add Ice Water: Gradually add ice water, one tablespoon at a time, until the dough comes together.
4. Chill the Dough: Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.
5. Roll and Pre-Bake: Roll out the dough on a floured surface, transfer to a pie pan, trim excess edges, and prick the base with a fork. Pre-bake at 375°F (190°C) for 10-12 minutes. Allow to cool.

Step 2: Make the Chocolate Filling

1. Combine Dry Ingredients: In a medium saucepan, whisk together sugar, cocoa powder, flour, and salt.
2. Add Wet Ingredients: Slowly whisk in milk and beaten egg yolks until smooth.
3. Cook the Mixture: Cook over medium heat, stirring constantly, until the mixture thickens (about 10-12 minutes). Do not let it boil.
4. Add Butter and Vanilla: Remove from heat, then stir in butter and vanilla extract until fully incorporated.
5. Cool Slightly: Allow the filling to cool slightly before pouring into the pre-baked crust.

Step 3: Assemble the Pie

1. Pour the Filling: Pour the chocolate filling into the prepared pie crust, smoothing the top with a spatula.
2. Add Optional Topping: If desired, top with whipped cream or meringue.

Step 4: Bake (if using meringue topping)

1. Meringue Topping: Beat the reserved egg whites with ¼ cup sugar until stiff peaks form, then spread over the filling.
2. Bake: Return the pie to the oven and bake at 375°F (190°C) for 10-12 minutes, or until the meringue is golden.

Step 5: Chill and Serve

1. Let the pie cool to room temperature, then refrigerate for at least 2-3 hours before serving.
2. Garnish with chocolate shavings if desired.