In a medium sized mixing bowl, combine the flour, baking soda, baking powder, and salt with a whisk, then set aside.
In a large mixing bowl, add softened butter and granulated sugar. Cream them together with an electric mixer until light and fluffy. Add in the egg, and mix until combined. Then add in the vanilla and heavy cream or milk, mixing until everything is well combined.
Add the dry ingredients into the wet ingredients gradually, mixing well between each addition. Scrape down the sides of the mixing bowl when needed.
Add green food coloring, a few drops at a time, and mix until you reach your desired shade of watermelon green.
Use a cookie scoop to scoop 1 ½ tablespoon portions of dough onto a parchment lined baking sheet. They can be close together, but not touching. Cover the tray with plastic wrap and chill for at least two hours, and up to overnight.
Line two sheet trays with a Silpat or parchment paper and preheat the oven to 375°F/190°C. Remove the cookie dough from the fridge.
Roll each cookie ball between your palms to form a smooth ball place them on prepared cookie sheets about 2 inches apart to allow for spreading.
Bake for 8-10 minutes until set and puffed (but not yellow). Let the cookies cool on the tray for 10 minutes, then transfer to a wire cooling rack to cool completely.
To make the frosting: Start by creaming butter in a large bowl. Slowly add the powdered sugar and Kool-Aid Powder and beat until smooth. Add the salt, vanilla extract, and heavy cream, and continue blending to get a smooth frosting. If needed, add a bit of red food coloring to get a deeper color.
Add the frosting to a piping bag fitted with a large closed star tip. Swirl rosettes onto each cookie, then top with mini chocolate chips to look like watermelon seeds.
Notes
Buy the right kind of Kool-Aid: Look for the drink mix in a small packet, not a large jar.
Chilling is important: To keep your sugar cookies thick and chewy, you definitely want to chill the cookies before baking them. If you skip this step, the cookies will turn out flat.
Avoid overcooking: The green-tinted cookie dough will turn yellow if the cookies are baked for too long. Pull these from the oven once they've puffed up and let them cool on the tray to set.
Use Gel Food Coloring: To get these bright, vivid colors, you want to use food coloring that has a lot of pigment and won't add moisture.
The frosting will be light pink after you add the Kool-Aid. If you want a darker watermelon color, add a few drops of red food coloring as well.
Swirl the frosting from the inside to the outside to create the pretty rosettes that you see in the photos here.
Be sure to add the chocolate chips before the buttercream sets up. If you wait too long, they won't stick.
To Store: Keep frosted cookies in a single layer in an airtight container in the fridge for up to one week.