First make the compote: Ideally you want the compote to macerate in the fridge for 12 hours. Or at least 4 hours at a minimum. Combine the plums, sugar, cinnamon stick (or powder) and toss them together in a bowl.
Zest and juice the orange and add to the plums. Give them all a good toss again and refrigerate for 4-12 hours. The longer the better.
Add the plums to a saucepan and cook over medium heat until they break down and become jammy, about 7-10 mins.
Stir frequently making sure the sugars don’t catch at the bottom. Taste and add more sugar if necessary. Cook for a few more mins if you add more. Remove the cinnamon stick and set aside.
Whisk together milk and eggs in a wide shallow bowl. Stir together the sugar and cinnamon powder in a small bowl.
Melt a cube of butter in a pan. Dunk a slice of bread in the milk-egg mixture and flip to coat both sides evenly. Fry in the butter until lightly browned on both sides.
Sprinkle the cinnamon sugar mix over one side and flip so the cinnamon sugar side caramelizes and browns. Keep the heat on low for this so the sugar doesn’t burn.
Repeat with the rest of the bread slices. Serve hot with the macerated plum compote spooned over the top.