Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, cream butter and sugar until light and fluffy (about 2–3 minutes).
Beat in the egg until fully incorporated.
Add vanilla bean seeds and watermelon extract; mix until evenly distributed.
In a separate bowl, whisk flour, baking powder, and salt.
Gradually mix dry ingredients into wet ingredients just until combined.
If using food coloring, fold in a tiny amount for a soft pastel pink.
Scoop dough into 1½-tablespoon portions and roll gently into balls.
Roll tops in sanding sugar if desired.
Bake for 10–12 minutes, until edges are set and centers look slightly soft.
Cool on pan for 5 minutes, then transfer to a wire rack to cool completely.