Step 1: Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease with butter.
Step 2: In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined.
Step 3: In a separate medium bowl, whisk the Greek yogurt, melted butter, eggs, and vanilla extract until smooth.
Step 4: Gently fold the wet ingredients into the dry ingredients using a spatula. Stop mixing when just a few streaks of flour remain.
Step 5: Toss the chopped cherry plums with a tiny bit of flour, then fold them into the batter. Do not overmix.
Step 6: Divide the batter evenly among the muffin cups, filling each about three-quarters full.
Step 7: Sprinkle the tops with turbinado sugar for extra crunch and place a small slice of plum on top if desired.
Step 8: Bake for 22-25 minutes, or until a toothpick inserted into the center comes out clean.
Step 9: Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.