Preheat the oven to 350°F (175°C) and line a 9x13-inch (23x33cm) baking pan** with parchment paper. Spray a bit of baking spray or few oil drops underneath the paper that it sticks to the pan. Set aside.
In a large mixing bowl, whisk flour, sugar, cocoa, baking powder, and salt to combine. Set aside.
In another large mixing bowl, whisk oil, buttermilk, eggs, and vanilla to combine, about 1 minute. Add hot water and whisk to combine, about 1 minute. Add the dry ingredients and whisk just until combined, about 1 minute. The batter will be thin. Then transfer the batter to the prepared baking pan and bake for 18-22 minutes or until a toothpick inserted into the center of the cake comes out clean with a few moist crumbs attached. Let cool for 10 minutes.
While the cake is cooling, start immediately with the frosting. Therefore, whisk the butter and cocoa until combined, and no lumps remain, about 1 minute. Gradually add the hot milk while whisking. Then add the vanilla and powdered sugar and whisk until smooth and combined, about 1-2 minutes.
Spoon the icing on top of the cake and spread evenly. Top with pecans and set aside for 15-30 minutes to allow the frosting to set. Serve immediately or store in an airtight container at room temperature for up to 3 days. Freeze for up to 3 months.