Preheat oven to 350℉. Line baking sheets with parchment paper for easy clean up.
If making the eggless version: in a small bowl mix milled flax seeds and water together and set to the side.
In a large bowl whisk together dry ingredients: almond flour, cassava flour, oat flour, maple sugar, baking soda, xanthan gum, pinch salt.
Add palm shortening and cut in with a pastry cutter until it looks like coarse sand.
**If making egg free version** Add flax egg mixture and pure vanilla. Cut in with a pastry cutter until the dough is fully combine and moist. Gather into a ball.**If making egg version** Add beaten egg, 2 tablespoon of water, and pure vanilla. Cut in with a pastry cutter until the dough is fully combine and moist. If it still looks slightly dry add an additional 1 tablespoon water. Gather into a ball. Flatten dough into a disc. Lightly dust a clean work surface with extra cassava flour. Place dough on surface and top with a large sheet of parchment paper. Begin rolling out dough. Start from the center working out in all directions until dough measures about ¼" in thickness.
Using cookie cutters cut out dough and place on a cookie sheet. If the cookies stick to the work surface, use an offset icing spatula to help loosen for easy removal.
Gather all the scraps of dough into a ball. Repeat steps 6-8 until the scraps of dough leftover are too small to cut into another cookie. I usually repeat this process 3-4 times.
These cookies do not spread during baking so you can place them pretty close together if you want. Bake 7-8 minutes until lightly browned. Allow to cool about 5 minutes on baking sheet. Then, use a spatula to transfer to a wire rack to finish cooling about 5-10 more minutes.
Prepare icing while cookies are baking. Add powdered sugar to a medium bowl. Add melted butter, pure vanilla, and 2 tablespoon almond milk. Use a fork or whisk to combine. Add remaining almond milk 1 tablespoon at a time until desired consistency is reached. You don't want it to be too runny, as it will make for messy decorations.
Leave icing as is or separate into different bowls and color as desired. Add to a small piping bag or small storage bag. Cut a small hole at the corner of the storage bag/end of the piping bag. Decorate cookies as desired.
After decorating cookies allow icing to dry completely. Depending on how much icing you use this will take about 1-2 hours. Store in an airtight container with a sheet of parchment paper between layers at room temperature.