Measure and blend fresh watermelon to create approximately 1 cup of pureed watermelon. Set aside.
Preheat the oven to 350°F (175°C). Line a cupcake tin with paper liners.
Cream together softened butter and sugar in a large mixing bowl using an electric mixer on medium speed until light and fluffy, about 3 minutes. The mixture should appear pale and creamy.
Add eggs one at a time, beating well after each addition, until the batter is smooth. Mix in vanilla extract.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt.
Gradually add dry ingredients to wet mixture in three additions, alternating with watermelon puree, starting and ending with dry ingredients. Mix until just combined. If desired, add a few drops of green food coloring for a pastel contrast.
Divide batter evenly into cupcake liners, filling each about two-thirds full. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean and the edges are lightly golden. Remove from oven and cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.