Place one rack in lower third of oven; place another rack in center of oven. Preheat oven to 350°.
To prepare Tan Brownies, place 6 tablespoons butter and brown sugar in a medium bowl; beat with a mixer at medium speed until light and fluffy. Beat in 2 eggs and 1 teaspoon vanilla. Weigh or lightly spoon 4.5 ounces (about 1 cup) flour into a dry measuring cup; level with a knife. Combine 4.5 ounces flour, baking powder, and 1/4 teaspoon salt, stirring well. Add flour mixture and pecans to sugar mixture, beating just until combined. Spoon into a 13 x 9–inch baking pan coated with cooking spray, spreading evenly with a knife or rubber spatula. Bake at 350° in lower third of oven for 15 minutes.
To prepare Black Brownies, melt chocolate and 4 tablespoons butter in a large microwave-safe bowl on HIGH for 1 minute or until melted, stirring after every 20 seconds until smooth. Add granulated sugar, stirring until well combined. Add 2 eggs, 1 teaspoon vanilla, and Guinness, stirring with a whisk until well combined. Weigh or lightly spoon 4.5 ounces (about 1 cup) flour into a dry measuring cup; level with a knife. Combine 4.5 ounces flour and 1/4 teaspoon salt, stirring well. Add flour mixture to chocolate mixture, stirring to combine. Pour mixture evenly over Tan Brownies.
Bake on the center rack at 350° for 25 minutes or until a wooden pick inserted into center comes out almost clean. Cool in pan on a wire rack; cut into squares.