Preheat oven to 175°C (350°F). Grease and flour a 9x13x2-inch baking pan to prevent sticking.

Melt butter in a large saucepan over medium heat. Stir in water and cocoa powder until smooth. Remove from heat and let cool slightly—it should still be warm when mixed with dry ingredients.

In a separate bowl, dissolve baking soda in buttermilk. Add beaten eggs and vanilla, then whisk in the slightly cooled cocoa mixture until smooth.

In a large bowl, whisk flour, sugar, cinnamon, and salt. Pour in the cocoa mixture and stir just until combined—don’t overmix. The thin batter is normal.

Pour batter into the prepared pan, smoothing the top. Bake for 30 minutes or until a toothpick inserted comes out clean.

While cake bakes, melt butter and sifted cocoa in a saucepan. Stir in buttermilk and heat until nearly boiling (5–7 minutes). Remove from heat, then mix in confectioners sugar, vanilla, pecans, and salt. Beat briefly with an electric mixer if needed.

As soon as the cake comes out of the oven, pour hot icing evenly over the top. Spread quickly with a spatula before it sets. Let cool 30–45 minutes before slicing.