Grease bottom and sides of a 9" glass pie plate with butter; set aside. Heat 8 tbsp. butter and brown sugar in a 1-qt. saucepan until sugar dissolves. Transfer butter mixture to a medium bowl; stir in ground wafers. Transfer mixture to pie plate; press into bottom and sides, using the bottom of a measuring cup to compress crust. Refrigerate for 20 minutes. Bake at 350 or 12 min until set; let cool.

Heat half-and-half in a 4-qt. saucepan over medium-high heat until it just begins to simmer; remove pan from heat.

In a large bowl, whisk together 2?3 cup sugar and cornstarch; add egg yolks and whisk until smooth.

Drizzle half-and-half into egg yolk mixture, whisking constantly, until smooth.

Return mixture to saucepan; heat over medium heat. Cook, stirring often, until bubbles rise to the surface and mixture is very thick, 3?4 minutes.

Remove pan from heat and add remaining butter and chocolates in small batches, whisking until smooth; stir in 1 1?2 tsp. vanilla.

Set a sieve over a medium bowl and strain chocolate mixture. Cover with plastic wrap, pressing plastic onto surface; refrigerate until set, about 4 hours.

Remove plastic wrap from chocolate filling and, using a rubber spatula, stir mixture until smooth. Spoon mixture into reserved crust, forming a dome, and smooth surface with the spatula.

Whipped Topping: In a large bowl, whisk confectioners sugar, vanilla, and whipping cream until stiff peaks form; spread on top of filling, forming a dome. Add chocolate sprinkles to top. Keep refrigerated until ready to serve.