Preheat the oven to 350 degrees.
Butter or spray only the bottom, not the sides, of a 13-by- 9-inch or 9-inch square baking pan.
Melt the butter and chocolate chips together in a double boiler or metal bowl over simmering water (or in a microwave, checking every 15 seconds). Remove from heat and allow mixture to cool a bit. Meanwhile, in another bowl, stir together the eggs, vanilla, sugar, and Purple Puree. Combine this purple egg mixture with the cooled chocolate mixture.
In a mixing bowl, stir together Flour Blend, cocoa powder, oats, and salt. Add this to the chocolate mixture and blend thoroughly. Mix in the chopped walnuts, if using, then pour the entire mixture into the baking pan.
Bake for 30 to 35 minutes, until a toothpick comes out clean. Allow to cool completely in pan before cutting the brownies and use a plastic or butter knife. Dust with powdered sugar, if desired.
Keeps for a week in the refrigerator, covered tightly.
Sneaky Chef Make-Ahead Recipe: Purple Puree.
If using raw spinach, thoroughly wash it, even if the package says "prewashed." Bring spinach or collards and water to boil in a medium pot. Turn heat to low and allow to simmer for 10 minutes. If using frozen blueberries, quickly rinse them under cold water to thaw a little, and then drain.
Fill the bowl of your food processor with the blueberries and cooked spinach, (or collards) along with the lemon juice and 1 tablespoon of water, and puree on high until as smooth as possible. Stop occasionally to push top contents to bottom. If necessary, use a second tablespoon of water to make a fairly smooth puree.
This amount of spinach and blueberries makes only about 1 cup of puree. Double the recipe if you want to store another cup of the puree. It will store in the refrigerator up to 2 days, or you can freeze 1/4 cup portions in sealed plastic bags or small plastic containers.