In a large bowl, sift the flour, add the baking powder and caster sugar, and mix to combine.
Separately, beat the eggs with a pinch of salt, add the yogurt, oil and vanilla extract, and give it a good stir.
Add the brown sugar and cinnamon to the plums, and use a spoon to mix well, so the plums can be well coated.
Mix the wet ingredients with the dry ingredients, and add the plums.
Use a spatula to incorporate everything.
Divide the batter between each muffin tin hole lined with paper cases.
Bake in the preheated oven at 200 degrees Celsius for 20-25 minutes until golden and well risen.
Leave to cool before serving.