For the pie crust:
In the food processor use the chopping blade and add pieces of the biscuits and run the processor till the biscuit is a little finer.
Add the butter cubes 1 by one as the processor is running through the feeding tube.
Run the processor until the mixture looks like wet sand.
Take the mixture out to the pie dish and spread till the dish is well covered with the biscuit and butter mixture. This is your piecrust.
Refrigerate it till needed. Preferably keep it covered with a cling film.
Now let us make our filling:
In a small bowl add the ½ cup egg yolks and beat them. Set aside.
In a small pan actually I prefer my kadhai/wok add the sugar, corn flour, coffee and stir it together.
Add the 1 cup of milk and whisk together.
Put the pan over low-flame and heat the mixture stirring continuously until the mixture is hot and starts bubbling at the sides.
Remove from heat and stirring continuously the hot milk mixture add the milk and yolk mixture. Mix well and fast. Be careful there is no need to spill the mixture or hurt yourself.
Transfer back the gas when well mixed and continue heating the moment it thickens up and bubbles remove it from the heat. It will be a thick pudding consistency.
Immediately add the chopped chocolate, vanilla essence and butter. Mix till it is well mixed and everything looks glossy and the chocolate is well melted.
Transfer to the pie crust and let it set and chill in the fridge.
After the first 15-20 minutes I placed a loose plate on the pie dish to keep myself out of dipping in the setting custard.
Chill for about 4-6 hours. (In our Indian heat 6 is more like it).
Cut into slices and serve with whipped cream ( if you wish. Since it was sweet enough for us I served it plain.