Preheat your oven to 350°F (175°C).
Blind bake the pie crust: Line the unbaked crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes. Remove parchment and weights, then bake for another 10-15 minutes until lightly golden. Let cool completely.
Prepare the filling: In a large bowl, cream together the softened butter and melted chocolate until smooth.
Gradually beat in the granulated sugar until well combined.
Add the egg yolks one at a time, beating well after each addition. Stir in the vanilla extract and salt.
In a separate clean, dry bowl, beat the egg whites with cream of tartar until stiff, glossy peaks form.
Gently fold about one-third of the beaten egg whites into the chocolate mixture to lighten it. Then, carefully fold in the remaining egg whites until just combined, being careful not to overmix.
Pour the filling into the cooled pie crust. Smooth the top.
Bake for 30-35 minutes, or until the edges are set and the center is still slightly wobbly. Do not overbake. Let cool completely on a wire rack.
Prepare the topping: In a cold bowl, whip the heavy cream with powdered sugar and vanilla extract until stiff peaks form.
Spread or pipe the whipped cream over the chilled pie.
Chill the pie for at least 4 hours, or preferably overnight, before serving.
