Preheat the oven to 350F. Begin by making the cinnamon cookies. Add the softened butter, brown sugar, and white sugar to a bowl. Use a stand mixer or hand mixer to mix for 3-4 minutes, until well combined. Scrape down the sides and add the egg, egg yolk, and vanilla. Mix until well combined.
Add the flour, cornstarch, cinnamon, baking soda, and salt. Mix on low until just combined. Add the white chocolate chips and pulse until well distributed. Set the dough aside and start on the streusel. For the streusel add the softened butter, brown sugar, cinnamon toast crunch, flour, cinnamon, and salt in a bowl. Use a stand mixer or hand mixer to mix until the streusel is well combined. The streusel should stick together when you squeeze it in your fist, but crumble when you run it through your fingers.
Add 1/4 cup of cinnamon toast crunch to a blender. Blend until its fine crumbs. Pour the crumbs into a bowl. Use a large cookie scoop (about 1/4 cup) to scoop out the cinnamon dough. Roll the dough in your hands into a ball, then drop it into the bowl of cinnamon toast crunch crumbs. Roll the dough ball around in the crumbs until it is coated all over.
Use a spoon or the back side of the cookie scoop to make an indent in the top of the dough ball. Add about 2 tbsp of streusel on top of the dough ball, in the indent. Repeat this process with the remaining dough.
Add up to 6 dough balls on a parchment lined baking sheet then transfer to the oven. Bake for 8-9 minutes. Let the cookies cool on the baking sheet for 10 minutes, then transfer to a cooling rack to cool completely.
While the cookies are cooling, make the icing. Add the powdered sugar, milk, and vanilla extract to a bowl. Whisk until smooth. When the cookies are cool, drizzle icing on top of each one and enjoy!