For the pudding: Heat 3 cups of the milk in a medium saucepan over medium heat until just barely simmering. Meanwhile, whisk the remaining 1/4 cup of milk and the cornstarch in a bowl until all the lumps are gone. Then whisk in the sugar, salt and egg yolks. Slowly pour about 1 cup of the hot milk into the yolk mixture while whisking, then add the contents of the bowl into the pot with the rest of the milk. Bring to a simmer while constantly whisking and simmer until thickened, about 2 minutes. Add the vanilla.
Divide the pudding into 2 large bowls. Add the chocolate to 1 bowl, stirring until melted and smooth. Add the butter to the plain vanilla bowl, stirring until melted.
Pour the chocolate pudding into the cookie pie crust, then spoon the vanilla pudding on top and gently spread it with an offset spatula so that it covers the chocolate layer. Use the offset to gently reach down to the chocolate layer and spin and flip it to swirl the puddings. Repeat all over the surface of the pie. Refrigerate until completely set, at least 4 hours.
For the raspberry whipped topping: Add the raspberries to a bowl and mash with a fork. Gently fold the mashed raspberries into the whipped topping until combined. Dollop on top of the pie slices. Garnish with fresh raspberries.