Preheat oven to 375°F. Prepare a 15 x 10-inch jelly roll pan by spraying lightly with cooking spray; dust with flour. Set aside.
In large bowl, whisk together flour, sugar, baking soda, ground cinnamon, and salt. In measuring cup, measure out buttermilk, and add vanilla and eggs.
Combine water, butter, and cocoa in a small saucepan; bring to a boil, stirring frequently. Pour into flour mixture. Beat with a mixer at medium speed until well blended. Add buttermilk mixture; beat well.
Pour batter into prepared pan. Bake for 17-18 minutes or until a wooden pick inserted in center comes out clean. Cool cake on a wire rack.
Frosting:
Meanwhile, combine 6 tablespoons butter, milk, and ¼ cup cocoa in a saucepan. Bring to a boil, stirring constantly. Remove from heat. Gradually stir in powdered sugar; stir in vanilla and pecans. Spread over hot cake. Cool completely in pan on wire rack.
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Notes
If desired, you can omit the pecans or substitute another nut of your choice.
If you don't care for the cinnamon, you can leave it out.
Storage: Leftover cake can be covered with plastic wrap and stored at room temperature for a day, or in the fridge for up to 4 days.