• Place the bowl and whisk attachment for your mixer into the freezer.

Make the crust.

• (The crust requires a brief baking. See the notes below for a no-bake option.)
• Preheat the oven to 325° F. Have a 9-inch pie plate ready.
• Grind the 3 cups vanilla wafer cookies into fine crumbs using a food processor. (If you don't have a food processor you can crush them in a bowl using a pestle or a heavy-bottomed glass.)
• Add the ¼ cup cocoa powder to the crumbs and pulse the food processor to blend them in. With the motor running pour in the 6 tablespoons butter, melted and run the food processor until the chocolaty crumbs are thoroughly saturated. 
• Press the mixture into the bottom and up the sides of the pie plate, using the bottom of a glass or a pastry tamper. It should be a firm, even layer over the entire bottom and sides. 
• Place the crust in the oven and bake for 15 minutes. Move the crust to a cooling rack. The crust will have puffed a little: use the glass or tamper to gently press it down and against the sides. Allow the crust to cool completely. 

Make the whipped cream:

• Assemble the chilled bowl and whisk on the stand mixer. Pour the 2½ cups heavy cream into the bowl. Beat the cream on medium-high for a minute. Stop the mixer; add in the ½ cup confectioner's sugar and the ½ teaspoon cream of tartar if using.
• (Cream of tartar will help whipped cream stay set for longer, but if you don't have any, don't worry about it.)
• Whip the cream for another 2 to 4 minutes, until stiff peaks form. 
• Remove 2 cups of the whipped cream and put them in the fridge. (This will go on at the end.) Transfer the rest of the whipped cream to a bowl and hold it aside for the filling.

Make the filling:

• Place the 6 ounces semi-sweet chocolate bar in a microwave-safe bowl. Microwave it in 30 second blasts, stirring after each one, until the chocolate is melted and smooth. Set it aside to cool—it must be at room temperature before it goes into the filling.
• Wipe out the mixer bowl. Add in the 16 ounces cream cheese, softened and the 1 pinch of salt; beat the cream cheese on medium until it's whipped and smooth.
• Add in the 1½ cups confectioner's sugar; beat it into the cream cheese until the mixture is smooth, scraping down the sides once or twice during mixing.
• The chocolate should be room temperature—once it is, scrape it into the mixing bowl. Run the mixer on medium-low, scraping the sides once or twice, until the chocolate is fully incorporated.
• Pour the ½ cup Baileys Irish Cream liqueur into the bowl and run the mixer on medium speed to incorporate it.
• Take the whipped cream you set aside in the bowl: use a rubber spatula to gently fold it into the chocolate-cream cheese mixture, using big strokes to pull the mixture from the bottom up and over the top. (This is to keep the mixture light and airy.)

Assemble the pie:

• Spoon the chocolate-Baileys filling into the cooled crust. It's a lot of filling and will be higher than the edge of the pie plate, but should be firm enough to stay in place. Use a rubber spatula to spread it evenly and smooth the top.
• Take the 2 cups of whipped cream from the fridge. Spread this evenly over the top of the pie filling.
• Loosely cover the pie with plastic wrap and place it in the fridge to chill and set for at least 4 hours or overnight.
• Once chilled, add Sprinkles to the top: you can use stencils to create shapes or just sprinkle them all over.
• Slice the pie and serve immediately. Store any leftovers covered in the fridge for up to 4 days.