Adjust your oven rack to the middle position and preheat the oven to 400°F. Prepare a quarter sheet pan by spraying it with baking spray or greasing and flouring it to prevent sticking.
In a large saucepan, combine canola oil, unsalted butter, cocoa powder, and water. Heat the mixture over medium heat, stirring often, until it comes to a boil. The chocolate should melt beautifully, creating a rich mixture.
While the cocoa mixture heats, whisk together in a large mixing bowl the flour, granulated sugar, baking soda, kosher salt, espresso powder, ground cinnamon, eggs, buttermilk, and vanilla extract. Make sure everything is well combined for even flavor throughout the cake.
Once the cocoa mixture has boiled, remove it from the heat. Slowly pour the hot mixture into the flour mixture, stirring gently to combine. Be sure to mix until fully incorporated and smooth, as this will affect the cake's texture.
Pour the batter into the prepped sheet pan, gently tapping it on the counter a few times to release any air bubbles that may have formed.
Place the pan in the oven and bake at 400°F for about 10 minutes. After that, rotate the pan and reduce the temperature to 375°F, baking for an additional 10 to 15 minutes. You’ll know it’s done when a toothpick inserted into the center comes out clean.
Once baked, transfer the cake to a cooling rack. Allow it to cool slightly while you prepare the frosting.
In a separate saucepan, bring the cocoa powder, unsalted butter, and milk to a low boil for the frosting. Stir in kosher salt and vanilla extract, combining well.
Remove the saucepan from heat and whisk in powdered sugar and chopped pecans until the frosting is smooth. Make sure you do this while the frosting is still warm.
Spread the hot frosting over the still warm cake, letting it soak into the cake for an incredibly moist texture.
Let the frosted cake cool for at least an hour before serving, allowing the frosting to set properly.