Prepare the Crust:

If not already done, pre-bake the pie crust according to its instructions. Let it cool completely.

Make the Chocolate Filling:

In a medium saucepan, whisk together sugar, cocoa powder, cornstarch, and salt. Slowly add the milk, stirring to avoid lumps.

Cook over medium heat, stirring constantly, until thickened and bubbly (about 8–10 minutes). Reduce the heat to low and cook for 2 more minutes.

Remove from heat. In a small bowl, whisk egg yolks. Gradually add a small amount of the hot chocolate mixture to the yolks to temper, then whisk the yolks back into the saucepan.

Return to heat and cook for 2 more minutes, stirring constantly. Remove from heat and stir in butter and vanilla extract.

Pour the chocolate filling into the cooled pie crust. Let it cool slightly.

Make the Meringue:

Preheat the oven to 350°F (175°C).

In a clean mixing bowl, beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually add sugar, one tablespoon at a time, beating on high speed until stiff, glossy peaks form.

Assemble the Pie:

Spread the meringue evenly over the warm chocolate filling, sealing the edges to the crust. Use the back of a spoon to create decorative swirls or peaks.

Bake the Pie:

Bake in the preheated oven for 10–12 minutes, or until the meringue is golden brown. Watch carefully to avoid burning.

Cool and Serve:

Let the pie cool to room temperature, then refrigerate for at least 2 hours before serving to allow the filling to set. Slice and enjoy!