Soak the millet and the buckwheat overnight or for an entire day.
Preheat the oven to 350°F (175°C).
Drain and rinse the grains and add them to the blender along with the apple, coconut, maple syrup, cacao powder, mineral water, and walnuts.
Blend until totally smooth, about 3 minutes.
Pour the batter into a mixing bowl, add the baking powder and the baking soda, and whisk just until incorporated.
Line your muffin tin with liners and fill each one with batter.
Bake for 35 minutes.
While the cupcakes are baking, make the frosting.
Place the cooked sweet potato, walnuts, dates, maple syrup, and cinnamon into the blender and blend until totally creamy and smooth, about 2 minutes.
Put the frosting in the fridge until the cupcakes are out of the oven and totally cool.
When the cupcakes are cool, spread a generous amount of frosting on each.