Preheat oven to 350 and heavily grease a bundt pan.
First prepare the cake batter. In a large bowl combine cake mix, vanilla pudding mix, greek yogurt, vegetable oil, eggs, and warm water and mix well. Stir in crushed cinnamon toast crunch cereal.
Prepare the cinnamon swirl by whisking together the white sugar and cinnamon in a small bowl.
Pour 1/3 of the cake batter into the pan. Sprinkle half of the cinnamon-sugar mixture on top of the batter. Next, spread another third of the batter on top of the cinnamon-sugar. (Pro Tip: grease a rubber spatula with cooking spray or butter, the batter will spread easier!) Sprinkle remaining cinnamon sugar on top of the batter, then top with the last third of the batter.
Bake for 40-45 minutes until an inserted toothpick comes out clean. Allow to cool for at least 30 minutes before turning cake out onto a cooling rack.
After cake has completely cooled, prepare the icing. Mix together cream cheese, vanilla, and powdered sugar. Add milk 1 tablespoon at a time until a pourable consistency is reached. Drizzle over cake, then top with cinnamon toast crunch cereal if desired. Store in airtight container at room temperature until ready to serve.