1. Make brownies Position a rack in the center of oven and preheat to 350 Line a 13 x 9 pan with aluminum foil so that foil extends 2 inches beyond the two long sides of pan. Butter bottom and sides of foil-lined pan.
2. In a medium sauce pan, combine butter and sugars. Over medium heat, stir mixture with a wooden spoon for 5-7 minutes, or until butter is melted.
3. Place chocolate chips and unsweetened chocolate in a food processor fitted with a metal chopping blade. Process chocolates for 15-20 seconds or until finely chopped. With motor running, pour hot butter mixture through feed tube and process for 15-20 seconds or until chocolate is completely melted, scraping down work bowl as necessary. Add eggs and vanilla and process for 10-15 seconds or until combined. Add flour and salt and process for 5-7 seconds, just until combined, scraping down work bowl as necessary. Add walnut halves and pulse about 10 times to incorporate them into mixture and to chop them slightly.
4. Scrape batter into prepared pan and smooth top with a rubber spatula. Bake brownies for 25-30 minutes or until a cake tester or toothpick inserted 2 away from center comes out slightly moist. Do not overbake brownies.
5. Cool brownies completely in pan set on a wire rack. Using two ends of foil as handles, lift brownies out of pan. Invert onto a flat plate or cutting board and remove aluminum foil. Reinvert brownies so they are top-side-up. Using a sharp knife, trim sides of brownies so that they are straight and smooth. Place brownies on a wire rack over a baking sheet.

Glaze

1. Make the bittersweet chocolate glaze Do not make the glaze until brownies are cool. In a medium saucepan over medium-low heat, bring cream to a gentle boil. Take pan off heat and add chocolate.. Let sit for 1-2 minutes to melt chocolate. Using a wire whisk, stir chocolate and cream until smooth. Strain glaze through a fine sieve into a small bowl to remove any air bubbles. Coo glaze for 5 minutes, until slightly thickened but still pourable. Pour glaze evenly over brownies allowing glaze to drip down sides onto baking sheet. Spread glaze with a small metal offset cake spatula, making sure that top and sides of brownies are completely covered.

Decoration

1. Decorate brownies In top of a double boiler over hot, not simmering water, melt milk chocolate, stirring often, until smooth. Transfer milk chocolate to a small paper cone or small plastic bag.
2. Repeat step 7 using white chocolate.
3. Cut a 1/16 opening at tip of both paper cones of plastic bags. (Trim plastic bags if they are too big and cumbersome.) Pipe thin straight parallel lines of milk chocolate at 1/2 intervals across top of brownies. Pipe tin straight lines of white chocolate in between lines of milk chocolate. At 3/8 intervals, draw the tip of a toothpick or metal skewer completely across brownies perpendicular to the piped chocolate lines, reversing direction in which you pull toothpick across brownie. Wipe toothpick tip clean between each pull. This will create a feathered design. Refrigerate brownies for 15 minutes, or until glaze is set. Cut into bars.