Preheat the oven to 350°F and spritz a 10-inch deep dish pie pan with cooking spray.
In a small bowl, toss together the chocolate cookie crumbs, melted butter and sugar. Press evenly into the pie plate.
Bake for 12-15 minutes or until set. Cool completely.
Melt the bittersweet chocolate with 2 Tbsp butter until smooth. This can be done in a double boiler over simmering water or in a microwave. In the microwave, heat in 20 second increments, stopping to stir each time. Repeat until smooth.
To make the custard: In a small bowl, mix together the sugar, cornstarch and salt.
In a separate bowl, whisk egg yolks together. Whisk beaten yolks into cornstarch-sugar mixture.
Add half and half and remaining 2 Tbsp butter to a heavy bottomed saucepan. Cook over medium-high heat until the butter has melted and the half and half is hot.
Whisk the sugar-egg mixture into the half and half then bring to a boil. Lower the temperature to medium to prevent scorching, whisking constantly for 1 minute after thickened.
Remove from the heat and whisk in the melted chocolate and vanilla.
Pour into a glass bowl, and press plastic wrap directly on top of the chocolate custard to prevent a film from forming. Chill completely for around 2 hours.
After chilling the chocolate custard, stir until smooth then spread into the pie shell.
Chill for 4-6 hours to allow the filling to set-up prior to cutting.
Top with fresh whipped cream and a sprinkle of cocoa powder before serving.