Preheat the oven and line the muffin tray with paper muffin cases.
Plums: Wash, dry, stone, and cut them into small pieces. Set them aside.
Dry ingredients: In a large bowl, mix flour, baking powder, cinnamon, a pinch of salt, and sugar using a fork.
Wet ingredients: In a separate bowl, mix the eggs, oil, and yogurt using the fork again. Make sure the wet ingredients are well combined.
Combine: Pour the egg mixture over the flour-sugar mixture. Mix with the fork until the ingredients are combined; don’t overmix; some streaks of flour in the batter are ok.
Fold in the plums with a rubber spatula or a spoon with just a few movements. Don’t overmix.
Bake: Divide the batter between the prepared paper-lined muffin cups. Bake in the middle of the oven for 20-22 minutes. The muffins should be golden brown, and a toothpick inserted in the center of a muffin should come out clean.
Rest for 5-10 minutes in the tray. Remove and cool completely on a wire rack.
Sprinkle with icing sugar before serving (optional).