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  In a large bowl, combine the flour, sugar, baking powder, and salt. Use a whisk to mix them.
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  Add the egg, vanilla, and softened butter to a medium bowl and whisk to blend.
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  Pour the wet ingredients into the dry ingredients and mix into cookie dough.
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  Wrap the cookie dough in plastic wrap and chill in the fridge for 45 minutes. This helps the dough to not spread while baking, and it helps soften the rice flour in your blend so the cookies don't have a gritty texture.
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  Preheat the oven to 350º F.
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  Remove the cookie dough from the refrigerator, and place it onto a silicone mat. Roll the dough to 1/4 thickness.
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  Use your cookie cutters to cut out the cookie shapes. Place them onto a parchment paper-lined baking sheet.
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  Bake the cookies for 10-11 minutes. The time may vary depending on the thickness and size of the cookies.
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  Remove the cookies and then them cool completely on a cooling rack. Your cookies need to be 100% cooled before you decorate them.

Royal Icing

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  Add the powdered sugar, egg whites, and cream of tartar to a stand mixer. Use the whisk attachment. Whip on low speed as the powdered sugar gets mixed in. You can use an electric mixer.
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  Mix the icing for 9 minutes. This is what will thicken this mixture into royal icing.

Decorating

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  Move some of the royal icing into small bowls and use food coloring to color each batch. I made a few shades of green icing.
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  If you plan to make sections of the same color as in the rainbow cookies pictured in the post, add royal icing to one section, then let that section dry. This will leave indentations or lines between the sections. You can see more examples of how to decorate in this way in my Gluten Free Halloween Cut-Out Cookies post
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  Allow the cookies to fully dry. The royal icing will harden to lock in the pattern. This will take 4-5 hours, longer if it is humid.