In a medium mixing bowl, combine vanilla pudding mix with cold milk, stirring to whisk out any lumps. Set aside to thicken for 5 minutes.
In the meantime, combine ½ cup heavy whipping cream and 3 tbsp powdered sugar. Whip to form stiff peaks, then fold the whipped cream into the thickened vanilla pudding.
Transfer the vanilla pudding mixture into a piping bag or Ziploc bag and set in the fridge.
Pour chocolate chips and the remaining heavy whipping cream into a microwave safe bowl and microwave for 45 seconds or until the cream is hot but not boiling.
Stir with a rubber spatula until the chocolate is melted and smooth. Transfer to another piping bag or Ziploc bag and set aside.
Lay half of the nilla wafers (flat side facing up) on a baking sheet lined with parchment paper or a silicone baking mat.
Cut off one end of the vanilla pudding bag, then squeeze a dollop of the nilla pudding onto each vanilla wafer. Lightly press another nilla wafer on top to sandwich the filling.
Cut off one end of the ganache bag and squeeze a dollop on top of each cookie sandwich. If needed, go back in with a spoon to smooth out the surface.
Transfer to the fridge to chill or enjoy immediately!