Preheat oven to 350F. Butter a 8-9" pie dish.
In a food processor, add the cookies to pulse until fine crumbs. Pour in the melted butter and pulse 2-3 more times, until very well combined. Press evenly into buttered pie dish, starting at the bottom and working up along the edges.
Bake for 8-10 minutes. If needed, use the back of a spoon to help press down any parts that have shifted or puffed up during baking. Turn the oven off.
In a large saucepan, combine the sugar, cornstarch, cocoa powder, and salt. Mix well with a whisk.
Place over medium heat.
While whisking, gradually pour in the whole milk and heavy cream.
Whisk continuously for 8-10 minutes over medium heat, until the mixture starts to bubble and is thickened. It is important that the mixture is fairly thick as this is what helps set the pie.
Remove from heat to stir in the chopped chocolate.
Stir in the butter, vanilla, and (optional) espresso powder.
Gently pour into the semi-cooled crust, smooth out the top with the back of spoon, and let cool at room temperature for 15-20 minutes. Transfer to the fridge to cool completely for 1 hour to overnight.