In a large bowl, use an electric mixer to beat the cold heavy whipping cream until stiff peaks form. Set aside in the refrigerator.

In a separate heatproof bowl, whisk together the egg yolks and granulated sugar. Place the bowl over a saucepan of simmering water (a bain-marie) and whisk constantly for 5-7 minutes, until the mixture is pale, thick, and has doubled in volume. Remove from the heat and allow it to cool for 10 minutes.

To the cooled egg mixture, add the room temperature mascarpone cheese, pistachio paste, and vanilla extract. Gently fold with a spatula until just combined. Gently fold in the prepared whipped cream in two stages until the mixture is smooth and uniform.

Pour the chilled espresso into a shallow dish. Quickly dip each side of a ladyfinger into the espresso, ensuring they don't become soggy.

Arrange a single layer of the dipped ladyfingers in the bottom of an 8x8 inch baking dish. Spread half of the pistachio cream mixture evenly over the top.

Repeat the process with a second layer of dipped ladyfingers and top with the remaining pistachio cream. Smooth the surface with a spatula.

Cover the dish with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow it to set completely.

Just before serving, dust the top generously with unsweetened cocoa powder and sprinkle with chopped pistachios. Slice and serve chilled.