Preheat oven to 400 degrees. Place butter in a large baking dish and insert dish into oven to allow butter to melt as it heats. When butter is melted remove dish from oven.
Chop watermelon into 1-inch chunks, removing all seeds. Set aside.
In a large mixing bowl, combine flour, sugar, and baking powder. Add in buttermilk, vanilla extract, lemon juice, and cinnamon. Stir until well combined and no clumps remain.
Add watermelon to the melted butter in the baking dish. Pour batter over the watermelon, covering each piece.
Sprinkle the top of batter with sugar. Place baking dish back in preheated oven and bake for 30 to 45 minutes, or until edges are golden brown and bubbly.
Notes
Helpful Tip: When serving a hot dessert with ice cream or whipped topping, allow dish to cool slightly so topping does not immediately melt.