Preheat the oven to 375°F—grease or line 12 cups of a muffin tin with paper liners.
Toss the fresh plum pieces in the extra tablespoon of flour in a small bowl.
To make the streusel, combine the flour, light brown sugar, and ground cinnamon in a small bowl to make the streusel. Stir the melted butter into the dry ingredients until a crumb forms. Set the streusel aside.
Mix the remaining flour, granulated sugar, light brown sugar, ground cinnamon, baking powder, and salt in a large bowl until combined.
Whisk the eggs, buttermilk, and vanilla extract in a medium bowl. Pour the wet mixture and {cooled} melted butter into the dry mixture. Mix the ingredients until combined and a batter forms—do not over-mix. Fold in the chopped almonds and fresh plum pieces.
Spoon the muffin batter between the 12 prepared muffin cups. Evenly sprinkle the cinnamon streusel over the top of the muffins. Chill the muffins in the refrigerator for 15-20 minutes before baking.
Bake the muffins at 375°F for 20-25 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Let the muffins cool in the muffin pan for 5-7 minutes before transferring them to a wire rack to cool completely.