01 - In a medium bowl, mix graham cracker or chocolate cookie crumbs with sugar. Stir in melted butter until evenly moistened. Press firmly into the bottom and up the sides of a 9-inch pie dish. Freeze for 15 minutes.
02 - Preheat oven to 350°F. Bake crust for 10 minutes, then allow to cool completely.
03 - Melt chocolate in a heatproof bowl set over a saucepan of barely simmering water, stirring until smooth. Remove from heat and cool to room temperature.
04 - Beat butter and powdered sugar with an electric mixer on medium speed until light and fluffy, about 2-3 minutes.
05 - Add cooled melted chocolate and vanilla extract to the butter mixture. Beat until well combined.
06 - Add eggs one at a time, beating for 3-5 minutes on medium-high speed after each addition until silky and light. This step is essential for a mousse-like texture.
07 - Spread chocolate filling evenly into the cooled crust and smooth the top. Refrigerate for at least 4 hours or until set.
08 - Whip heavy cream, powdered sugar, and vanilla together to soft peaks. Spread or pipe over the chilled pie.
09 - Garnish with chocolate shavings if desired. Slice and serve chilled.