Combine butter and granulated sugar in a large bowl and use an electric mixer to beat together until creamy and well-combined.
1 cup unsalted butter, 1 ½ cups granulated sugar
Add egg and vanilla extract and stir again.
1 large egg, 1 teaspoon vanilla extract
In a separate bowl, whisk together flour, baking powder, and salt.
2 ½ cups all-purpose flour, ¾ teaspoon baking powder, ¾ teaspoon salt
Gradually add dry ingredients to wet ingredients (I like to do this in 3 or 4 parts) until the flour has been completely incorporated. This is a somewhat dry dough and may seem crumbly, but scrape the bottom of the bowl periodically and the dough will come together if you continue mixing (assuming you have measured your flour properly).
Once dough is clinging together and all of the flour has been absorbed, divide it in half, placing each half in its own bowl. Then, working with one half, divide that dough almost in half again, allotting slightly more to one portion than the other.
To the largest portion of dough, add red or pink food coloring and stir until you have a vibrant pink color (adding more food coloring as needed). Add 2 Tablespoons of mini chocolate chips and stir until combined. Set aside.
Red or pink food coloring, 4 Tablespoons mini chocolate chips
To the second largest portion of dough, add green food coloring (adding more coloring as needed) and stir until you have a vibrant green color. The third/smallest portion of dough should not be colored.
Green food coloring
Form the pink section of dough into a log that is about 12” long. Make it slightly wider than it is tall and round one wide end and flatten the other (if at any point the dough is too sticky to handle without difficulty, simply cover it and place in the refrigerator for 15-20 minutes before proceeding). Place the log flat-side down
Take the un-colored dough and place it on a lightly floured surface or between two pieces of wax paper. Roll with a rolling pin until you have a rectangle that is large enough to cover the pink section. Carefully lift the dough it and use it to cover the rounded edge of the pink log (make sure this completely covers the pink section). Use a sharp knife to trim away any excess dough.
Repeat step 8 with the green dough, making sure you roll it large enough to completely cover the pink and white dough. Cover the un-colored dough, then trim any excess dough as needed.
Cover the surface of the dough with wax paper and chill in the refrigerator for at least 30 minutes before proceeding. While dough chills, preheat oven to 375F (190C).
Remove dough from refrigerator and use a sharp knife to slice log into slices between ⅛-¼” thick. Place on a parchment paper lined baking sheet, spacing cookies at least 2” apart. Use a knife to carve slightly into the pink part of the cookie, making a slightly crescent shape (this is illustrated in the video and photos above).
Press remaining chocolate chips point-down into the pink portion of the slices to make distinct watermelon “seeds”.
Transfer to 375F (190C) preheated oven and bake for 9 minutes. Allow to cool completely on baking sheet before enjoying.
Store in an airtight container at room temperature for up to one week.
Making dough in advance
The dough may be prepared and assembled up to one week in advance. Store it covered in the refrigerator and slice and bake as desired!