Prepare and bake brownie mix according to package directions in an 8" square pan. Let cool completely in pan on a wire rack. Cut brownies into 3 dozen squares.
Beat butter and shortening at medium speed with an electric mixer until creamy. Add brown sugar, beating until smooth. Add eggs, egg yolk, and vanilla, beating until blended.
Combine flour and next 3 ingredients; gradually add to butter mixture, beating just until blended after each addition. Stir in chocolate morsels. Cover and chill dough 1 hour.
Increase oven temperature to 350 degrees F. For each cookie, scoop a level 1/2 cup dough onto parchment paper-lined baking sheets; place cookies 3" apart. Using your fingers, make an indentation in mound of dough; place 1 brownie square in center. Press chocolate chip dough around brownie, reshaping dough to form a ball. Repeat procedure with remaining dough and 10 brownie squares, placing no more than 6 cookies on a baking sheet. Chill 15 minutes.
Bake at 350 degrees F for 18 to 20 minutes or until lightly browned and cookies look set. Cool 2 minutes on baking sheets; remove to wire racks, and cool completely.
Yield: 11 jumbo cookies. Note: This recipe uses only a portion of the baked brownies. Enjoy remaining brownies, or freeze for later use.