Preheat the oven to 350°F (175°C) and line a muffin tin with cupcake liners.
Whisk together the dry ingredients in a medium bowl.
Beat butter and sugar until fluffy, then add eggs, vanilla, and watermelon puree.
Gradually add the dry ingredients to the wet mixture.
Fill cupcake liners 2/3 full with batter.
Bake for 18-20 minutes or until a toothpick comes out clean.
Cool cupcakes completely.
For frosting, beat butter, then gradually add powdered sugar. Add lime juice, zest, and heavy cream for consistency. Frost cupcakes.