How to Make It
Preheat the oven to 350°F. Butter and flour a 13-by-9-inch baking dish.
Cook the quinoa in a pot of boiling salted water until it is tender, 10 to 15 minutes. Drain the quinoa in a sieve and let it cool to room warm. Place the quinoa back in the pot and stir in the butter and milk over low heat, until the butter is melted. Reserve.
In the bowl of an electric mixer, beat together the eggs and sugar on high speed until the mixture has almost tripled in volume 8 to 10 minutes.
In a medium bowl, whisk together the flour, mace, baking powder and 3/4 teaspoon salt, then beat the flour mixture into the egg mixture until combined. Stir in the quinoa mixture (batter with be quite wet). Pour the batter into the buttered dish.
Stir together the remaining 1/2 cup sugar and 1 teaspoon mace, then sprinkle the mace-sugar evenly over the batter. Bake the cake until a tester comes out clean, about 30 minutes. Let cook to room temperature, then cut into slices and serve with whipped cream.