Preheat oven to 180C/350F/Gas 4. Line a cookie sheet with parchment paper and set it aside.
Using an electric handheld or stand mixer, cream together the butter, powdered sugar, vanilla, and egg until combined and creamy.
Sift or whisk the flour, baking soda, and cream of tartar in a large bowl.
At low speed, gradually add the dry ingredients to the wet ingredients until just combined.
Then use your hands to mix to a dough.
Cover and refrigerate for 30 minutes.
Dust your surface with flour and knead the cookie dough a couple of times before rolling to about ½"/1cm thick.
Cut out 12 cactus with a cactus-shaped cookie cutter and place them onto a cookie sheet.
Bake for 8 minutes or until lightly golden brown.
Leave cookies to cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely before decorating.
To make the royal icing, use an electric handheld or stand mixer to combine the egg whites, cream of tartar, vanilla, and powdered sugar until stiff peaks form.
Scoop out 1 cup of icing into a bowl and add a few drops of neon green gel coloring. Mix to the desired shade of green. This will be used to pipe the lines on the cactus.
Place the neon green icing into a piping bag fitted with a round no.2 piping tip.
Add a few drops of leaf green to the remaining icing and mix to the desired shade of green. This will be used to cover the base of the cactus cookies.
Scoop about ¼ cup of the leaf green icing into a piping bag fitted with a round no.2 piping tip.
Add 2-4 teaspoons of water into the remaining leaf green icing bowl and mix to thin the icing and make it slightly runnier. Pour the icing into a squeeze bottle.
Using the leaf green piping bag, outline each of the cookies.
Using the thinned icing, fill in the cactus outline.
Leave the cookies in a cool, dry place to dry for 24 hours.
Using the neon green piping bag, pipe a line around the main section of the cactus, and then around the arms.
Pipe broken lines down the centers of the cactus body and arms.
Pipe little hearts on some of the broken lines.
Allow the icing to dry for 2 hours before enjoying!