Rinse 1 cup of quinoa thoroughly under cold water using a fine-mesh sieve until the water runs clear.

Combine rinsed quinoa, 2 cups water, and 1/2 tsp salt in a medium saucepan. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes, or until all water is absorbed and quinoa is fluffy.

Remove from heat and let stand, covered, for 5 minutes. Fluff with a fork and spread on a plate or baking sheet to cool completely. You will need 1 1/2 cups for the cake.

Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans. Line the bottoms with parchment paper.

In a large bowl, whisk together the all-purpose flour, cocoa powder, baking soda, baking powder, and salt.

In a separate large bowl, combine the granulated sugar and light brown sugar. Add the eggs, vegetable oil, and vanilla extract. Beat with an electric mixer on medium speed until light and fluffy, about 2-3 minutes.

Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix on low speed until just combined, being careful not to overmix.

Gently stir in the hot coffee (or water) until smooth. The batter will be thin.

Fold in the cooled cooked quinoa until evenly distributed.

Divide the batter evenly between the two prepared cake pans.

Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs, but not wet batter.

Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire rack to cool completely. Ensure cakes are entirely cool before frosting.

In a large bowl, using an electric mixer, beat the softened butter on medium speed until light and fluffy, about 2-3 minutes.

Gradually add the sifted powdered sugar, cocoa powder, and salt, beating on low speed until combined, then increase to medium-high and beat until smooth.

Slowly add the milk (or heavy cream) and vanilla extract. Beat until light and fluffy.

Pour in the slightly cooled melted bittersweet chocolate and beat until well combined and the frosting is smooth and creamy. If the frosting is too thick, add a tiny bit more milk; if too thin, add more sifted powdered sugar.

Once the cakes are completely cooled, place one cake layer on a serving plate or cake stand. Spread about 1/3 of the frosting evenly over the top.

Carefully place the second cake layer on top. Frost the top and sides of the entire cake with the remaining frosting.

Optionally, decorate with chocolate shavings, sprinkles, or fresh berries.