CRUST: Preheat the oven to 350° F/175° C. Break the shortbread fingers into pieces and put them in the bowl of the food processor and pulse until you have fine crumbs. Add the sugar and melted butter and pulse until the crumbs are all wet. Press the crumb mixture into the bottom of the prepared pie plate, pressing up the sides and forming a rounded or squared off top edge. Bake in the preheated oven until golden, about 8 minutes. Set aside on a wire rack and leave the oven on.
FILLING: In the bowl of the stand mixer, whip the egg yolks and lemon zest at high speed until fluffy, about 5 minutes. Slowly add the condensed milk and continue to whip until thick, about 3 or 4 minutes. Reduce the mixer speed to low and slowly add the lemon juice, mixing just until combined. The mixture will start to thicken as soon as you add the juice. Pour the mixture into the crust.
BAKE: Bake until the filling has just set, about 10 minutes. Cool on a wire rack, then refrigerate. Freeze for 15 to 20 minutes before serving.
WHIPPED CREAM: Chill the mixing bowl. Using the wire whisk attachment, whip the cream and confectioners sugar until you have stiff peaks. Serve each slice of pie with a dollop of cream.