Crust: Preheat oven to 350F degrees. Butter a 9-by-2-inch cake pan (if you only have a 9-by-1.5-inch pan, as is the standard size, that will work fine too), then line with two 17-by-2-inch strips of parchment, forming an X in center and leaving a 2-inch overhang. Butter the parchment.
Place 1 sheet of phyllo on a work surface (I used parchment paper) with the long side facing you; keep remaining phyllo covered with plastic wrap. Lightly brush entire surface with melted butter, then sprinkle some of the granulated sugar on right half of the phyllo. Fold the left half over to enclose sugar. Brush top lightly with more butter. Press the folded phyllo into the pan, buttered-side down, leaving a 1/2-inch overhang on one side; gather and crumple dough slightly as you go to make fit and create a ruffled edge. Repeat with remaining phyllo sheets, overlapping to completely cover bottom of pan and create a 1/2-inch overhang on all sides. Bake until golden, about 20 minutes. (Watch your crust the first couple minutes of baking. If the sides start to fold into the crust of sink down the side, just carefully pull them back up and over the lip of the pan.) After removing pie from the oven, if it has risen too high, place a bowl into the pan over the crust. The crust will crunch and flatten just a bit, don't worry if it stays puffy. Let cool 10 minutes. Cook's note: If using a dark, non-stick pan, lower oven temp to 340F degrees and baking time will be closer to 15 minutes.
While the crust is baking, set a pot of water to heat on the stove, heat to a slow boil and maintain heat until time to make the filling.
<span class="mceItemHidden" data-mce-bogus="1">Brush melted chocolate over bottom of crust. the crust might break a bit as you do this, but no worries, just do what you can. Refrigerate while you complete the mousse.</span>
Filling: In a heat-proof bowl set over (not in) the pot of simmering water, whisk together the egg yolks, granulated sugar, and salt until sugar is dissolved. Whisk in 85% chocolate until melted. Remove from heat.
Beat 1 cup of the cream until medium-stiff peaks form (do not add sugar or vanilla here). Whisk one-third of this cream into the chocolate mixture. Gently but thoroughly fold in remaining whipped cream. This may take a minute to fully incorporate, but don't worry, just keep stirring. Pour mixture into cooled crust, letting mousse get into folds, if there are any. Refrigerate at least 3 hours.
When ready to serve, fill a large bowl or your sink with hot water (about 2-inches), place cake pan into water for about 10-15 seconds, then place pan on counter. Gently try to pull up on the parchment straps, if pie will not come up, place in hot water another 5-10 seconds. Pull pie out of pan and discard parchment strips. Place on serving plate.
Beat remaining 1 cup cream with sugar and vanilla until soft peaks form. Dollop onto pie, sprinkle with chocolate shavings and serve. (To make the chocolate shavings, just use a sharp paring knife on a chocolate bar and scrape out pieces to place on the whipped cream.)
Notes: If you're hesitant about using dark chocolate, substitute the 85% chocolate in the filling with 60% bittersweet chocolate.