In a medium bowl, toss the plum slices with 1 1/2 tablespoons sugar and set aside.
In a large bowl, whisk together the remaining 3 tablespoons sugar and the flour, baking powder, baking soda and salt.
In a medium bowl, whisk together the buttermilk, the eggs and 1/4 cup melted butter until combined. Make a well in the center of the dry ingredients; pour the buttermilk mixture and pecans into the well and whisk until incorporated.
Brush a nonstick griddle or skillet with butter and heat over medium heat. Working in batches, spoon 1/4 cup batter onto the griddle for each pancake. Gently press 3 plum slices into each pancake, lower the heat to medium-low and cook until golden brown on the bottom and the top is bubbling near the edges, 3 to 4 minutes. Flip and cook for 2 more minutes. Serve with the maple syrup.