Preheat oven to 400. Add the butter and brown sugar to a large bowl or stand mixer. Mix for 1-2 minutes, until the sugar and butter is totally incorporated and fluffier. Add eggs and vanilla and mix until combined.
Add flour, cornstarch, baking soda, and salt. Mix until all dry ingredients are combined. Add white chocolate chips and mix them in.
Using a large (1/4 cup) cookie scoop, scoop out dough balls onto a lined baking sheet. Flatten them slightly with your hand. Bake at 400 for 9-11 minutes, until they are barely set and the bottoms are golden brown (the cookies may still look a little doughy, but they will set up!).
While the cookies cool, make the frosting. Add the butter to a bowl and whip for 1 minute. Add the cream cheese and whip an additional 2 minutes. Add vanilla, salt, and powdered sugar. Mix for 3 minutes until the frosting is light and fluffy. If you want a thicker frosting, add more powdered sugar (i added 4 cups).
When the cookies are cooled, pipe frosting on top of each cookie then sprinkle on crumbled Cinnamon Toast Crunch. Enjoy!