For the pudding: Whisk together the cornstarch, eggs and egg yolks in a heatproof bowl. Combine the heavy cream, milk, sugar, salt and vanilla in a medium heavy pot. Cook over medium heat, stirring, until it begins to steam, then whisk in the peanut butter. Continue cooking, stirring, until it begins to bubble and the peanut butter is incorporated, then turn off the heat. Temper the eggs by adding a couple ladles of the hot cream and peanut butter mixture to the eggs, whisking vigorously. Pour the egg mixture into the pot, whisking, and turn the heat back on medium. Cook, stirring continuously, until the mixture has thickened, about 5 minutes. Pour into 8 heatproof serving glasses, then cover and refrigerate until cool and set, a few hours or up to overnight.
To decorate: Knead maroon food coloring into half the marzipan for the beets and knead orange food coloring into the remaining marzipan for the carrots. Form the marzipan into vegetable shapes. Cut the rosemary sprigs and use them as stems. Sprinkle the crushed cookies on top of the pudding as "dirt," then top with the marzipan vegetables.