1. Preheat Oven
   Preheat the oven to 350°F (175°C).

2. Melt Butter
   Melt the butter in a saucepan over medium heat.

3. Mix Dry Ingredients
   In a bowl, whisk together the sugar, flour, and cocoa powder.
   Gradually add the dry mixture to the melted butter, stirring to prevent any lumps.

4. Add Milk
   Slowly add the milk to the saucepan while stirring until the mixture is smooth.

5. Cook Until Thickened
   Cook the mixture for 5-7 minutes, stirring constantly, until it thickens. Once thickened, remove from heat.

6. Temper Egg Yolks
   In a separate bowl, whisk the egg yolks. Add a spoonful of the hot chocolate mixture to the yolks to temper them, then gradually stir the yolks back into the saucepan.

7. Finish Cooking
   Return the saucepan to the heat and cook for 2-3 minutes, stirring constantly. Stir in the vanilla extract and remove from heat.

8. Assemble the Pie
   Pour the chocolate filling into the pre-baked pie crust and spread evenly.

9. Prepare Meringue
   In a clean mixing bowl, beat the egg whites and cream of tartar until soft peaks form.
   Add the sugar and continue beating until stiff peaks form.

10. Top with Meringue
    Spread the meringue over the chocolate filling, making sure to seal the edges of the pie crust.

11. Bake
    Bake the pie for 10-12 minutes, or until the meringue is golden brown.

12. Cool and Serve
    Allow the pie to cool completely before serving. This allows the filling to set properly.

Serving Suggestion:

Enjoy this rich and velvety chocolate pie with a dollop of whipped cream or fresh berries for an added touch.