2 - 3ozdark chocolate, finely chopped or shaved with a vegetable peeler
Combine 3/4 cup + 2 Tbsp granulated sugar, the light brown sugar, salt and eggs in a heat proof bowl set over a pot of simmering water. Allow mixture to heat over simmering water, whisking constantly, until it registers 160 degrees on a candy thermometer, about 8 minutes.
Remove from heat, add chocolate chips and stir until chocolate has melted, then mix in vanilla. Allow mixture to rest until it reaches 75 degrees, stirring occasionally (you can put it in the fridge to cool faster).
In the bowl of an electric stand mixer fitted with the paddle attachment, whip butter on medium-high speed until pale and fluffy. Add in cooled chocolate mixture, switch to the whisk attachment and whip on high speed until pale and fluffy, about 6 minutes longer.
In a large mixing bowl, whisk together cocoa and remaining 2 Tbsp granulated sugar, then while whisking, slowly pour in about 1/2 cup of the heavy cream and stir until smooth.
Add remaining 3/4 cup heavy cream, then using an electric hand mixer set on high speed, whip until stiff peaks form. Fold whipped cream mixture into melted chocolate chip/butter mixture.
Pour into prepared pie crust and smooth into an even layer. Tent with foil (sealing foil around edges) and chill in refrigerator overnight.
Before serving, in a mixing bowl whip 2 1/2 cups heavy cream until soft peaks form. Add in granulated sugar and whip until stiff peaks form.
Spread over set pie and garnish top with chocolate. Cut into slices and serve chilled. Store in refrigerator.