In a stand mixer, cream together butter and sugar. Add egg and vanilla. Combine flour, baking soda, and salt. (Confession - I don't mix this in a separate bowl.)
Add flour to butter and sugar mixture. Mix well. Divide dough in half. Set one half aside. On a lightly floured surface, roll out half the dough.
Cut into perfect circles and place on a cookie sheet lined with parchment paper. Cook at 350 degrees for 8 to 10 minutes.
Royal Icing
Combine meringue powder and warm water until dissolved. Beat for about 1 minute. Add powdered sugar and corn syrup. Mix at medium speed for about five minutes. Remove a small amount of royal icing to a small bowl and add a little bit of water - drop by drop - until it's the consistency you want. Don't try to get all the royal icing to the same consistency at the same time.
Icing The Cookies
We rolled out enough dough to make almost three dozen tennis balls. We cut out the dough into circles and used the side of a butter knife to make light indents in the dough to guide us with our white lines later when our icing was done. To make the icing, I kept some icing white for the tennis ball lines and tinted the rest of the royal icing with lemon yellow and kelly green gel colors from Wilton. Use a tiny bit at a time until it's perfect.
When our coloring was perfect, I started by filling in the white lines first.
Next, I did a thin outline around the outside of the cookie with the green icing and then flooded the interior portion. My kids love to ice cookies and I let them. Perfection isn't the goal. They will look great, I promise!