Preheat the oven to 180C / 350F
Remove the pips from the plums and cut them into 8ths (smaller pieces). Place in a small bowl and mix in the 2 Tbsp of Demerara sugar and cinnamon. Set aside.
Sift the flour, baking powder, and bicarb into a bowl and then mix in the sugar.
In a measuring jug whisk together the eggs, sunflower oil, vanilla, yoghurt, lemon zest, and juice until smooth.
Add the wet ingredients to the dry and gently stir being careful not to overmix. Add the plums and fold through the batter until just incorporated.
Divide the mixture evenly between 12 paper-lined muffin cups and sprinkle a little extra sugar over to create a crunchy top. Bake for 20 – 25 minutes until golden brown and springy to the touch.