Place diced watermelon in a food processor or blender and puree until completely smooth. Measure out ¾ cup of the watermelon puree and set aside.
1 cup fresh watermelon flesh, diced
In a large mixing bowl, combine the flour, baking powder, and salt. Whisk together until evenly distributed.
1 cup fresh watermelon flesh, diced
In a separate bowl, beat eggs with sugar using an electric mixer or whisk until pale and slightly thickened, about 2 minutes. Add melted butter, milk, vanilla, and watermelon puree. Mix until smooth.
1 cup fresh watermelon flesh, diced
Gradually pour the wet mixture into the dry ingredients, folding gently with a spatula until just combined. Do not overmix; the batter will be slightly lumpy.
1 cup fresh watermelon flesh, diced
Line a cupcake tin with paper liners and evenly distribute the batter among 12 cups. Bake in a preheated oven at 350°F (175°C) for 20-25 minutes, or until a toothpick inserted into the center comes out clean and the cupcakes are golden around the edges.
1 cup fresh watermelon flesh, diced
Remove cupcakes from the oven and allow them to cool in the tin for 5 minutes. Transfer to a wire rack and let cool completely before frosting or serving.
1 cup fresh watermelon flesh, diced