In a large bowl, whisk together the milk, oil, eggs, sour cream, and vanilla until smooth. Add the cake mix and cornstarch. Mix on medium speed for 2 minutes, just until the batter is smooth and combined. Do not overmix.
Fill cupcake liners about two-thirds full. Tap the pan gently on the counter to release air bubbles. Bake until the centers are set and a toothpick comes out mostly clean. Let the cupcakes cool completely.
While the cupcakes cool, beat the butter until smooth and fluffy, about 2 minutes. Add the powdered sugar gradually, mixing until combined. Mix in the vanilla and 2 tablespoons of cream, then beat until light and pipeable. Fold in the crushed cookies by hand, adding more cream if needed.
Pipe the frosting onto the cooled cupcakes and finish with movie theater toppers before serving.