Add the almond milk in a pot together with the lemon zest and the vanilla. Bring it to a boil and take the milk away from the fire. Add the amaranth and polenta flour and stir continuously in order to make a smooth and without lumps. Add the ghee or butter and stir again. Set aside and let it cool for a while.
Preheat the oven onto 160°C/325°F.
Meanwhile add the sugar and the egg in a bowl and with a hand mixer mix until it becomes a fluffy cream. Add the rest of the ingredients and stir well.
In the egg-sugar-mixture add the polenta mix and mix with the hand mixer until you get a smooth, homogeneous cake batter.
Grease a cake pan of 7'', transfer the batter into it and move it into the oven. Bake for 50 minutes, then put the temperature to 180°V/350°F for the remaining 10 minutes. The top of the cake should be lightly golden-brown.
Take the cake out of the oven and let it cool out.