Line the 7-inch springform pan with parchment paper. For the graham cracker crust, put the graham crackers in a food processor and crush them, then stir in the orange zest and juice. Set the fruit of the orange you removed the zest from aside for later. Transfer the graham cracker mixture to the springform pan and press it down into the base. Transfer the springform pan to the refrigerator and let the crust chill.
Gradually heat the butter, egg, egg yolk, corn starch, orange juice, and sugar in a saucepan on the stove, making sure it doesn't start to boil. Once all of the ingredients are combined and a smooth, thick mixture has formed, pour the orange curd into an ice cube tray. Transfer the ice cube tray to the freezer and freeze the orange curd for 1 hour.
Bring the milk, sugar, agar agar powder, and corn starch to a boil in a saucepan on the stove, then let the mixture simmer for about 2 minutes, stirring continuously.
Remove the rest of the peel from the orange you set aside previously and cut the orange into slices. Place the slices in the springform pan, alternating between placing full and halved slices around the outer edge of the cake, then place 1 full slice in the center.
Use a ladle to spoon the milk pudding mixture into the springform pan, covering the bottom half of the orange slices. Transfer the springform pan to the refrigerator and chill for 5–10 minutes.
As soon as the milk mixture has set, remove the springform pan from the refrigerator and distribute the cubes of frozen orange curd on top of the first layer of milk pudding.
Pour the rest of the milk pudding mixture on top, transfer the springform pan to the refrigerator, and chill for 30 minutes. Once the cake has chilled, carefully flip the cake over onto the graham cracker crust and remove the ring.
