In a saucepan, heat the milk and sugar on medium heat, while continually stirring with a wooden spoon to keep the sugar from burning on the bottom of the pan.
When the sugar has dissolved, turn the heat down to low and add in the baking soda. Gently boil this mixture for about 1 minute, constantly stirring and watching that it does not bubble and foam over the top of the pan. It will burn easily so keep stirring and you will know it is ready when turns a light golden colour.
Remove the pot from the heat and stir in the butter until it has melted. Whisk in the psyllium husk, vigorously mixing to prevent lumps forming. Leave the pot to cool to lukwarm temperature for 5 or 10 minutes.
Sift the flours, cream of tartar and salt, stirring them into the liquid in about three portions. Lastly stir in the vanilla and combine well
Use a (tsp size) cookie scoop to form balls and place the cookies 3 inches apart on a parchment lined baking sheet. Flatten the tops with a fork. They will spread out quite a lot in the oven.
Bake the cookies for 7-10 minutes (depending upon size) or until the edges have a faint golden colour.
Cool the cookies on a baking sheet for 5 minutes, then gently remove them with an egg flipper and place on a cake rack to cool completely.
Store in an airtight container for up to a week.