Preheat the oven to 350°F (175°C). Line a muffin tin with cupcake liners. Measure and blend fresh seedless watermelon until smooth to make approximately 1 cup of watermelon puree.
In a large mixing bowl, cream together the softened butter and granulated sugar using an electric mixer until the mixture is pale and fluffy, about 2-3 minutes. The texture should be smooth and light when ready.
Add eggs one at a time to the butter and sugar mixture, beating well after each addition. Continue until the batter is smooth and slightly thickened, with a glossy appearance.
Mix in the vanilla extract and watermelon puree until fully incorporated, resulting in a bright pink, slightly chunky batter. Scrape down the sides of the bowl as needed.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing on low speed or folding until just combined. The batter should be smooth and uniform.
Spoon the batter evenly into the cupcake liners, filling each about two-thirds full. Place the muffin tin in the oven and bake for 20-25 minutes, or until the cupcakes are golden around the edges and a toothpick inserted into the center comes out clean. Allow to cool before serving.
