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.
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.