In a heatproof bowl over a pot of simmering water, combine granulated sugar, brown sugar, eggs, and salt. Whisk constantly for about 8 minutes until the mixture reaches 160°F on a candy thermometer.
Stir in the chocolate chips until melted, then mix in the vanilla. Let the mixture cool to 75°F (you can place it in the fridge to speed up the cooling).
In a stand mixer with the paddle attachment, beat the butter on medium-high until pale and fluffy. Once the chocolate mixture has cooled, add it to the butter, switch to the whisk attachment, and whip on high for 6 minutes until light and airy.
In a separate bowl, whisk together cocoa powder and sugar, then gradually whisk in about 1/2 cup of heavy cream until smooth. Add the remaining cream and use a hand mixer to whip until stiff peaks form. Gently fold the whipped cream into the chocolate mixture.
Pour the filling into the prepared pie crust and smooth the top. Cover with foil and refrigerate overnight.
Before serving, whip heavy cream until soft peaks form, then add sugar and continue whipping until stiff peaks form. Spread the whipped cream on top of the pie, garnish with chocolate, slice, and serve chilled. Store leftovers in the fridge.