Prep: 1. Preheat oven to 350 degrees and grease a 9x9 baking pan. Make: 1. Melt butter in heavy saucepan over low heat. Once melted, whisk in sugar, then chocolate chips. 2. Remove from heat and whisk in eggs and vanilla. Sift in cocoa powder and arrowroot powder and stir vigorously (batter should be well mixed and have a slight sheen) 3. Bake for 25 minutes, or until a toothpick stuck in the center comes out clean.