In a small saucepan, combine 12 ounces semisweet chocolate and the 1/2 cup butter. Cook and stir over low heat until melted and smooth. Cool slightly.
Preheat oven to 350 degrees F. Line a 13x9x2-inch baking pan with foil, extending foil over the edges of the pan. Butter foil; set aside.
In a large bowl, combine the 6 eggs and the 1-1/2 cups sugar. Beat with an electric mixer on medium speed until light and lemon-colored.
In a medium bowl, combine the 1-1/2 cups flour, the baking powder, and salt. Add flour mixture to egg mixture; beat until well mixed, scraping side of bowl occasionally. Gradually add melted chocolate mixture and the 1 tablespoon vanilla; beat well. Spread evenly in prepared pan; set aside. Wash beaters thoroughly with soap and warm water; dry beaters.
In another large bowl, combine cream cheese and the 6 tablespoons butter. Beat on medium speed until light and fluffy. Add the 3/4 cup sugar; beat until fluffy. Add the 3 eggs, the 2 tablespoons flour, and the 2 teaspoons vanilla; beat well. Carefully spoon cream cheese batter evenly over the chocolate batter.
Bake in preheated oven for 35 to 40 minutes or until cream cheese layer appears set in center when gently shaken. Cool in pan on a wire rack (center may fall slightly as brownies cool).
In a small saucepan, combine the 3 ounces chocolate and the shortening. Cook and stir over low heat until melted and smooth. Drizzle chocolate mixture over cooled brownies. Let stand until set. Using the edges of the foil, lift the uncut brownies out of the pan. Cut into bars. Store leftovers in refrigerator. Makes 32 bars.
To Store: Place bars in a single layer in an airtight container; cover. Store in the refrigerator for up to 3 days.