This crowd-pleaser is moist, dense, and ultra-chocolaty. Lift it out of the pan with its parchment-paper liner, so the shiny chocolate glaze is free to drizzle down the sides.
Cake: Preheat oven to 375 degrees. Lightly butter a 9-by-13-inch cake pan. Line with parchment, leaving a 2-inch overhang on long sides. In a large heatproof bowl, whisk together flour, granulated sugar, baking soda, salt, and cinnamon.
Melt butter in a saucepan over medium-low. Whisk in cocoa, then 1 cup water. Increase heat to medium-high and bring to a boil, whisking occasionally. Pour over flour mixture and stir to combine. Stir in eggs, buttermilk, and vanilla.
Pour batter into prepared pan; smooth top with an offset spatula. Bake until a tester inserted in center comes out clean, 22 to 24 minutes.
Icing: In a small saucepan, bring butter, cocoa, and cream to a boil, stirring occasionally. Remove from heat; whisk in vanilla and confectioners' sugar. Let stand until warm before using.
Transfer cake in pan to a wire rack; let cool 15 minutes. Pour glaze over cake. (For a more elegant presentation, remove cake from pan using parchment and place on a serving platter before glazing.) Sprinkle with pecans and let cool before serving.