Use a medium-size pot and place an inch of water in it, over medium heat. Whisk the eggs along with 1/2 cup of sugar in the water, using a large heat-proof bowl that will fit over the pot holding the water (without touching the water). With the bowl placed over the simmering water, whisk the eggs continuously until they turn thick and fluffy, and a string of the mixture will hold its shape on top of the custard. This takes about 5 minutes.
Using a large mixing bowl, add the softened butter, the rest of the sugar, along with the salt and beat with a hand beater or a stand mixer on medium-high speed until the mixture is fluffy. Add the chocolate and 1 1/4 teaspoons of the vanilla while beating. Be sure and scrape down the sides with a rubber spatula as needed. Add the custard and keep beating until the filling is thick, smooth, and fluffy. This takes about 5 minutes.
Fill the pie crust with the mixture, cover it with plastic wrap and chill until it sets, 2 to 3 hours. Once the bottom has set, you ca make the whipped cream topping by beating the cream with the powdered sugar and remaining 1/4 tsp of vanilla. Beat the cream until it forms soft peaks. Spread the whipped cream over the pie and garnish with chocolate shavings.
Using a larger bowl for cooking the custard witll keep your hand further from the heat and the eggs will cook faster.