In a large bowl, whisk together flour and salt. Cut in the cold butter with a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter. Gradually add ice water, 1 tablespoon at a time, mixing until the dough just comes together. Do not overmix.

Gather the dough into a ball, flatten it into a disc, wrap in plastic wrap, and refrigerate for at least 30 minutes.

Preheat oven to 375°F (190°C). On a lightly floured surface, roll out the dough into a 12-inch circle. Carefully transfer the dough to a 9-inch pie plate and crimp the edges. Prick the bottom of the crust with a fork.

Line the crust with parchment paper and fill with pie weights or dried beans. Blind bake for 15 minutes. Remove parchment paper and weights, and bake for another 5-7 minutes, or until lightly golden brown. Let cool completely.

Melt the chopped bittersweet chocolate in a heatproof bowl set over a saucepan of simmering water (double boiler), stirring until smooth. Remove from heat and set aside.

In a large bowl, beat the softened butter and granulated sugar until light and fluffy.

Beat in the egg yolks one at a time, until well combined.

Stir in the melted chocolate, vanilla extract, and espresso powder (if using). Mix until smooth.

In a separate clean bowl, beat the egg whites until stiff peaks form. Gently fold the beaten egg whites into the chocolate mixture until just combined.

Pour the filling into the cooled pie crust and spread evenly.

Refrigerate the pie for at least 4 hours, or until firm. The pie needs to be thoroughly chilled for the filling to set properly.

Before serving, top the pie with whipped cream and chocolate shavings.