Puree 1 cup of cubed watermelon in a blender until smooth, then measure out 1/2 cup and set aside. Dice the remaining watermelon into small 1/4-inch pieces and pat dry with paper towels.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt until evenly combined.
In a separate medium bowl, whisk together the buttermilk, watermelon puree, eggs, melted butter, and vanilla extract until smooth and well blended.
Pour the wet ingredients into the dry ingredients and gently fold together using a rubber spatula until just combined. Small lumps are fine — do not overmix.
Gently fold in the diced watermelon pieces, being careful not to break them up too much. Let the batter rest for 5 minutes to thicken slightly.
Heat a griddle or large non-stick skillet over medium-low heat and lightly grease with butter or oil. Test the heat by sprinkling a few drops of water — they should sizzle gently.
Pour 1/4 cup of batter onto the griddle for each pancake, spacing them about 2 inches apart. Cook until bubbles form across the surface and edges look set, about 3-4 minutes.
Carefully flip the pancakes and cook for another 2-3 minutes until golden and cooked through. The second side will brown slightly faster due to the watermelon sugars.
Transfer cooked pancakes to a plate and cover loosely with foil to keep warm while you cook the remaining batter, adding more butter or oil to the griddle as needed.
Serve immediately while warm, topped with your favorite toppings like whipped cream, honey, fresh watermelon slices, or a dusting of powdered sugar.
