In a medium saucepan, whisk the sugar with the cornstarch and salt. Gradually whisk in the milk until smooth and no pockets of cornstarch remain. Add the vanilla bean and seeds. Set the saucepan over moderately high heat and bring the pudding to a boil, whisking constantly. Reduce the heat to moderate and cook, whisking constantly, until thickened, about 7 minutes.
Strain the pudding into a bowl. Whisk in the egg yolks one at a time until thoroughly incorporated. Return the pudding to the saucepan and cook over moderately high heat, stirring constantly, until thickened again, about 2 minutes. Remove from the heat and scrape the pudding into a glass bowl. Whisk in the vanilla extract. Press a piece of plastic wrap directly onto the surface of the pudding and refrigerate until chilled, at least 4 hours or overnight.
Preheat the oven to 350°. Spread the macadamias in a pie plate and bake for about 5 minutes, until lightly browned. Let cool completely. Coarsely chop the macadamias and transfer to bowl. Add the rice cereal and toss. Coat a baking sheet with vegetable spray.
In a heavy medium saucepan, stir together the sugar, water and corn syrup and bring to a boil over moderately high heat. Reduce the heat to moderate and cook without stirring until a deep amber caramel forms, about 8 minutes. Remove from the heat and add the toasted nuts and cereal; gently swirl the pan so that the nuts and cereal are completely coated. Scrape the mixture onto the prepared baking sheet and let cool until hardened, about 30 minutes.
Transfer the brittle to a sturdy resealable plastic bag. Using a rolling pin, gently crush the brittle into bite-size pieces.
Whisk the pudding until smooth and spoon into bowls. Top with whipped cream and a sprinkling of brittle and serve.