Preheat the oven to 375° Farenheight. Line a muffin pan with baking liners. Set aside.
In a medium-sized bowl whisk together the flour, sugar, baking powder, and salt. Set aside.
In a separate bowl mix together the butter, eggs, and buttermilk until everything is well combined.
Slowly mix the wet ingredients into the dry ingredients until everything is well combined. Fold in the chopped plums.
Evenly divide the batter filling each muffin cavity up to the top.
Sprinkle the turnbanio sugar over the top of each muffin.
Bake for 25-30 minutes. Allow them to cool for 5-10 minutes.