1. Preheat oven to 375°F (190°C). Roll out pie dough into a 9-inch dish, prick the bottom, line with parchment and weights, and blind bake for 12–15 minutes. Remove weights and bake 5 more minutes until lightly golden. Let cool.

2. In a saucepan, whisk together sugar, cocoa powder, cornstarch, and salt. Gradually add milk, whisking until smooth. Cook over medium heat, stirring constantly until it thickens and bubbles.

3. In a separate bowl, beat egg yolks. Slowly whisk in a bit of the hot chocolate mixture to temper. Return to saucepan and cook another 2–3 minutes until glossy and thick.

4. Remove from heat. Stir in butter and vanilla extract. Pour filling into baked crust and smooth the top.

5. In a clean mixing bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar and beat until stiff, glossy peaks form. Add vanilla and cornstarch (if using).

6. Spread meringue over hot filling, sealing it to the crust edges. Create peaks with the back of a spoon.

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

8. Cool the pie at room temperature for 1 hour, then refrigerate at least 3 hours before slicing.