Preheat the oven and prepare the baking dish.

Crush the chocolate sandwich cookies and combine with melted unsalted butter to form a crust.

Press the crust mixture into the bottom of the prepared baking dish.

Melt the chopped bittersweet chocolate and combine it with unsalted butter, whiskey or water, cocoa powder, and vanilla extract in a bowl.

In a separate bowl, beat the egg yolks with brown sugar and granulated sugar until combined.

Gradually add the chocolate mixture to the egg yolk mixture and mix well.

In another bowl, whisk the egg whites with kosher salt until stiff peaks form.

Fold the beaten egg whites into the chocolate mixture gently.

Pour the combined mixture into the prepared crust and bake in the oven.

Allow the pie to cool completely.

For the topping, whip the heavy cream with powdered sugar and vanilla extract until soft peaks form.

Spread the whipped cream over the cooled pie and drizzle with Irish cream, if desired.