Add ice cream, milk, cinnamon, vanilla, and brown sugar (if using) to blender. Tip: For extra thickness, freeze your glass for 5 minutes first.

Blend on low for 15 seconds, then increase to high until smooth (about 30 seconds). Tip: Stop blender to scrape sides if needed.

Add 2 crumbled cookies and pulse 3-4 times just to incorporate. Tip: Leaving some cookie chunks adds texture.

Pour into glasses, top with remaining crumbled cookie, and serve immediately.