Make the pumpkin pie custard
Preheat the oven to 325°F. Add the milk, eggs, sugar, pumpkin pie filling, and a pinch of salt to a blender and blend until smooth. Arrange 4 (6-ounce) or 6 (4-ounce) ramekins in a baking or casserole dish (with sides at least 2 inches high). Distribute the custard evenly among the ramekins, leaving about ¼ inch of headroom at the top. Place the baking dish in the oven and carefully pour warm water into the dish until it reaches about ½ inch from the tops of the ramekins. Bake until the centers are slightly jiggly, 50 minutes to 1 hour. Carefully remove the custards from the oven and from the baking dish. Let cool to room temperature, then cover tightly with plastic wrap and refrigerate for at least 3 hours and up to 24 hours.
Make the corn flake crumble
While the custard is cooling, preheat the oven to 300°F. Combine the corn flakes, butter, sugar, salt, and cinnamon in a large bowl and use your hands to mix and break up the corn flakes until the mixture resembles granola. Spread the mixture on a rimmed baking sheet and bake until browned and toasted, 3 to 4 minutes. Let cool.
Serve
Top the chilled custards with the corn flake crumble just before serving.