• Preheat oven to 350°F. Grease and flour (or generously spray) a 13×18-inch half sheet pan or 12×17-inch jelly roll pan with sides. A larger pan gives the classic thin Texas-style cake. (A 9×13-inch pan will work, but the cake will be thicker.)
• In a large mixing bowl, combine cake mix, brownie mix, eggs, water (or buttermilk mixture), oil, vanilla, and cinnamon (if using). Mix for about 2 minutes until smooth and well combined.
• Pour batter into prepared pan and spread evenly.
• Bake for 25 to 32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Start checking at 25 minutes; do not overbake.
• While the cake is baking, make the frosting.

Frosting

• In a medium saucepan, melt butter over medium heat. Whisk in cocoa powder and milk until smooth.
• Bring to a gentle boil, stirring constantly, and cook for 1 to 2 minutes.
• Remove from heat and whisk in confectioners' sugar, vanilla, and salt until smooth and glossy. The frosting should be pourable but slightly thick. Add a splash of milk if needed.
• As soon as the cake comes out of the oven, pour warm frosting evenly over the hot cake. Spread gently if needed.
• Immediately sprinkle with toasted pecans, lightly pressing them into the frosting so they adhere.
• Let the cake cool completely in the pan (about 1 to 2 hours). The frosting will set into a glossy, slightly crackly layer.