Combine the diced plums with 1/2 cup sugar in a bowl. Let the plum mixture sit at room temperature for 30 minutes; the sugar will draw the juices out of the fruit & create a syrup.
Put bread cubes on a baking sheet. Bake bread briefly, just to dry it out a little. Let cool.
In a large bowl, beat the eggs, then whisk in the rest of the sugar, milk, half & half, salt, liqueur, 5 spice powder & cinnamon.
Add the bread cubes, folding gently, so all the bread is coated with egg mixture, then add plum mixture with all the syrup that has accumulated and gently mix again.
Allow mixture to sit at least 1/2 an hour, to let bread absorb the egg mixture & plum juices.
Butter 10 ramekins (custard cups). Divide bread/fruit mixture between them & sprinkle with the coarse sugar if using.
Bake for about 30 minutes or until center of the pudding wiggles slightly when you shake the cups & the top is golden brown.
Remove from oven & cool. Serve at room temperature, with whipped cream if desired.
Recipe Notes
Alternately you could use a 9 x 13-inch pan instead of the individual ramekins.