In a medium saucepan set over medium-low heat, whisk together the granulated sugar, brown sugar, cornstarch, gelatin, salt, and milk. Continue to stir constantly until the mixture is smooth and warmed through.

Add the chocolate chips and chopped dark chocolate to the saucepan. Stir gently until the chocolate is fully melted and the mixture begins to thicken. Allow the filling to come to a gentle boil, then boil for exactly one minute, stirring constantly. Remove from heat and stir in the vanilla extract.

In a separate bowl, lightly beat the reserved egg yolks. Slowly drizzle approximately 1 cup of the hot chocolate mixture into the yolks, whisking continuously to prevent curdling. Once combined, pour the tempered yolks back into the saucepan with the remaining chocolate mixture and stir well to incorporate.

Pour the chocolate filling into the pre-baked deep-dish pie crust, smoothing the surface with a spatula. Set aside while preparing the meringue.

In the clean bowl of a stand mixer or using a hand mixer, combine the six egg whites and cream of tartar. Beat on medium speed until frothy. Gradually add the sugar and increase the speed to high, continuing to beat until stiff, glossy peaks form.

Spread the meringue over the warm chocolate filling, ensuring it extends to the edges of the crust to seal completely. Use the back of a spoon to create peaks and swirls for visual texture.

Place the pie in a preheated 350°F (175°C) oven. Bake for approximately 15 minutes, or until the meringue is golden brown on top. Remove from the oven and allow to cool at room temperature for 30 minutes.

Transfer the pie to the refrigerator and chill for a minimum of 3 hours before slicing and serving. This allows the filling to fully set and the flavors to develop.