Preheat oven to 350°F (180°C) and grease an 8-inch (20 cm) square metal baking pan. Set aside.
In a large bowl, whisk together amaranth flour, cocoa powder, potato starch, flax seeds, baking soda, and salt.
In a medium bowl, whisk sugar, coconut milk, coconut oil, water, lime zest, and lime juice until blended.
Add the coconut milk mixture to the flour mixture and stir until just blended.
Spread batter evenly in prepared pan.
Bake in preheated oven for 27 to 32 minutes or until a tester inserted in the center comes out with moist crumbs attached. Let cool completely in pan on a wire rack.
To serve, spread coconut cream over cooled cake or simply serve it alongside.