Cream together unsalted butter and sugar until light and fluffy, about 3–4 minutes. Add egg and vanilla extract, mixing until fully combined.

In a separate bowl, whisk together flour and salt, then gradually add to the wet mixture until smooth.

Transfer dough onto a floured surface and roll to about ½ inch thick. Cut out circles, then cut each in half for watermelon slice shapes. Place on a parchment-lined baking sheet.

Preheat oven to 350°F (175°C) and bake for 10–12 minutes until edges are golden brown. Cool completely before decorating.

In a mixing bowl, combine warm water, meringue powder, and Karo syrup. Beat until smooth. Gradually add sifted powdered sugar until creamy.

Divide icing into bowls and color with gel food coloring appropriate for watermelon. Stir until vibrant.

Decorate cooled cookies, piping colored icing to resemble watermelon flesh and rind. Add dots of black icing for seeds.

Allow decorated cookies to set for about an hour before serving.