Add the milk and 3 cups of the cereal to a large bowl and stir to combine. Let the mixture steep for 1 hour in the fridge, then strain the cereal out and discard it, reserving the milk (there should be 4 1/2 cups).
Add 4 cups of the cereal milk to a large saucepan and warm over medium heat until steaming (do not let boil), about 6 minutes. Meanwhile, whisk together the sugar, cornstarch, egg yolks, 1/2 teaspoon salt and the remaining 1/2 cup cereal milk.
Slowly incorporate the warm cereal milk into the egg mixture, whisking constantly to keep the eggs from scrambling. Once combined, pour the mixture back into the pot and turn the heat to medium. Continually whisk the mixture until it thickens up to the consistency of pudding, about 7 minutes.
Remove the pudding from the heat and stir in the vanilla. Transfer to a large bowl and cover with plastic wrap pressed against the top of the pudding to prevent a skin from forming. Chill the pudding completely, at least 4 hours and up to 1 day.
Meanwhile, preheat the oven to 400 degrees F. Spread the remaining 1 cup cereal evenly on a baking sheet and drizzle with the agave. Bake until the agave melts and the cereal sticks when pinched, about 5 minutes. Remove from the oven and let cool. (The cereal garnish will harden as it cools.)
Spoon the pudding into bowls and top with the candied cereal pieces and sprinkles.
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