Preheat fan-forced oven to 160C or 320F. Position the oven rack in the lower third of your oven.
Line non-stick large baking trays with baking/parchment papers.
Using an electric stand mixer, cream the butter and stevia until light and fluffy.
Add the whole egg, egg yolk, agave syrup, rice milk and vanilla extract into the large bowl of a standing mixer. Beat and mix well until light and fluffy with the stand mixer.
In another large mixing bowl, shift in the gluten free all-purpose flour, corn starch, potato flour, xanthan gum and salt. Whisk and mix well.
Gradually add the flour mixture prepare in step 4 into the wet ingredients mixture prepared in step 2, stir and mix well on low speed until you get a smooth dough. Add 1 more tablespoon of rice milk if required. Dough should be smooth, sticky, easy to pipe and stays in shape when piped.
Prepare a piping bag fitted with an open star nozzle. Fold down the top of the piping bag and pipe the cookie dough all the way near the nozzle. Unfold the piping bag and fill around 2/3 full. Push the cookie dough all the way down and twist the bag where the cookie dough ends.
Pipe each cookie dough ½ inch apart, onto the lined baking trays.
Bake in the oven for 12 to 15 minutes or until the cookies are lightly brown around the edges.
Allow the cookies to cool on the baking trays for 5 minutes before removing to a cooling rack.
Once the cookies are completely cooled, store in an air-tight container.