1. Preheat the oven to 375°F (190°C) and prepare the pie crust in a 9-inch pie dish. Line with parchment and baking weights. Blind-bake for 15 minutes, remove weights, and bake for another 5 minutes. Let it cool completely.

2. In a medium saucepan, whisk together sugar, cornstarch, cocoa powder, and egg yolks. Slowly add milk while whisking.

3. Cook over medium heat, stirring constantly, until the mixture thickens to a pudding-like consistency. Remove from heat, stir in butter and vanilla extract.

4. Pour the hot chocolate custard into the cooled pie crust.

5. In a mixing bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, continuing to beat until stiff peaks form. Add vanilla extract and mix briefly.

6. Spoon the meringue over the hot custard, spreading it to the edges to seal completely. Use the back of a spoon to create peaks.

7. Bake at 350°F (175°C) for 12–15 minutes or until the meringue is golden brown.

8. Let the pie cool at room temperature for 1 hour, then chill for at least 3 hours before serving.