Adjust oven rack to middle position and preheat oven to 400°F. Spray a quarter (9x13-inch) sheet pan with baking spray or use the grease and flour method.
In a large saucepan, combine the oil, butter, cocoa, and water. Heat just to boiling, stirring often.
½ cup canola oil, ½ cup unsalted butter, ¼ cup unsweetened cocoa powder, 1 cup water
While the cocoa/butter mixture heats, whisk the flour, sugar, baking soda, salt, espresso powder, cinnamon, eggs, buttermilk, and vanilla together in a large mixing bowl.
2 cups all-purpose flour, 2 cups granulated sugar, 1 teaspoon baking soda, ½ teaspoon kosher salt, 2 teaspoons espresso powder, ½ teaspoon ground cinnamon, 2 large eggs, ½ cup buttermilk, 1 teaspoon pure vanilla extract
When the hot cocoa mixture just begins to boil, remove from the heat and slowly pour the hot mixture into the flour/sugar mixture. Beat the wet and dry ingredients together using a rubber spatula, wooden spoon, or with an electric mixer until fully incorporated and smooth.
Pour the batter into the prepared sheet pan and gently tap the pan on the counter a few times to release any air bubbles in the batter.
Bake at 400°F for 10 minutes then rotate the pan, reduce the temperature to 375°F, and bake an additional 10-15 minutes, or until a toothpick inserted in the middle comes out clean.
Transfer the cake to a cooling rack.
While the cake cools, make the fudge frosting: in a large saucepan, bring the cocoa, butter, and milk to a low boil. Add the salt and vanilla and stir well. Add the powdered sugar and pecans.
¼ cup unsweetened cocoa powder, ½ cup unsalted butter, ¼ cup whole milk, ¼ teaspoon kosher salt, 1 teaspoon pure vanilla extract, 3½ cups powdered sugar, ½ cup chopped pecans
Remove the saucepan from the heat and use a whisk to combine the ingredients together until smooth. Spread the hot frosting over the still warm cake.
Let the frosted cake cool long enough for the frosting to set before serving, at least 1 hour.