Preheat oven to 350 degrees F. Line the bottom of a 10" round cake pan with parchment paper and grease the sides with butter.
In a small pot, combine quinoa with 1 1/2 cups water. Bring to a boil, reduce to medium heat, and simmer uncovered for 10 minutes. Remove from heat and cover with a lid for 15 minutes.
Blend quinoa, olive oil, eggs, milk, and vanilla extract together in a high-speed blender until the mixture is completely smooth.
In a large mixing bowl, whisk together sugar, cocoa powder, baking powder, baking soda, and salt. Ensure any clumps of cocoa powder are broken down.
Stir quinoa mixture into sugar mixture until batter is fully incorporated.
Fold chocolate chunks and walnuts into batter and pour into the prepared pan. Even out the batter with a spatula.
Bake in the oven for 35-40 minutes until the center feels stable. Check the center with a toothpick if you are unsure. The toothpick won't be entirely clean, but won't be covered in batter either.
Allow the cake to cool entirely. It's normal and expected that the cake will sink a touch, due to its moist, fudgy nature.
Serve the cake with a light dust of powdered sugar and top with fresh fruit!
Chef's Tip: Always slice chocolate desserts with a heated knife for a clean cut. Do this by running the knife under hot water and wiping the water away before each cut.