1. To make panna cotta, heat oven to 300 degrees. Spread cereal on a baking sheet and bake until toasty, about 12 minutes. While still warm, transfer to large bowl or container and add milk and cream. Stir to combine and let steep 45 minutes. (Dessert will get too starchy if it steeps longer.).
2. Strain into a microwave-safe bowl or a saucepan, pressing to extract liquid. (Discard soggy cereal or eat it.) Add salt and brown sugar, and heat just until milk is hot enough to dissolve sugar, watching carefully. Stir gently to dissolve sugar. Ladle 1/4 cup milk mixture into a small bowl and mix in gelatin. Set aside 5 minutes, then whisk soaked gelatin back into remaining milk mixture.
3. Divide mixture among 8 ramekins or silicone molds. Refrigerate until set, about 2 hours. If using ramekins, cover and reserve until ready to serve. If using molds, freeze 1 hour and pop out onto wax paper (not parchment), then refrigerate until ready to serve.
4. Meanwhile, make topping, if you like: heat oven to 275 degrees. Put 3/4 cup cereal in a large bowl and crush lightly with your hands. In a small bowl, stir together milk powder, sugar and salt. Sprinkle mixture over crushed flakes and add melted butter. Toss to coat cereal evenly. Spread on a baking sheet lined with parchment paper (or a nonstick baking mat) and bake 20 minutes, or until deep golden brown. Remove from pan and set aside to cool. Serve immediately or store in an airtight container up to 1 week.
5. When ready, serve cold panna cottas in ramekins or turn out onto plates. Sprinkle generously with corn flake topping.