Bring the water to a boil in a medium saucepan. Add the amaranth, butter and salt and bring to a boil again. Reduce to low and simmer, covered, 20 to 25 minutes. Remove from the heat.

Stir in the maple syrup, cinnamon, nutmeg and vanilla until smooth. Garnish with tablespoons coconut flakes (if using) and walnuts.