Preheat the oven to 350℉ (180℃). Spray a 9x13 inch metal cake pan with non-stick cooking spray, or lightly grease and dust with cocoa powder. Set aside.
In a very large bowl, sift together the flour, cocoa, baking powder, baking soda and salt. Then whisk together until the cocoa powder is evenly distributed throughout the flour. Set aside.
In a separate large bowl, whisk together the oil, sugar, eggs, buttermilk and vanilla. Keep whisking until you no longer see lumps of egg. Then whisk the almost boiling water into the mixture. Go carefully so that the hot water doesn't splash.
Pour the wet ingredients into the dry ingredients. Whisk together until there are no lumps. You can also use an electric mixer for this. Be sure that there are no clumps of flour at the bottom of the bowl.
Pour the batter into the prepared pan (it will be thin). Bake in the middle of the preheated oven for about 35-40 minutes or until the top looks set and the sides are starting to pull away from the pan. An inserted toothpick should come out clean or with a few moist crumbs but no gooey cake batter.
Cool the cake (in the pan) for about 30-60 minutes.
Taking a wooden spoon or large straw, poke holes in the cake. Each hole should be about ¾ -1 inch apart (about 1½-2 cm) and go about ⅓ to ½ down into the cake.
If the caramel is cold, warm it up in the microwave for 45 seconds on medium power, then stir. Repeat until the caramel is warm enough to pour. Alternatively, warm up the sauce in a small saucepan over low heat. Measure out ½ cup and pour over the cake. Be sure to get the caramel into the holes. Place the cake in the fridge until ready to serve.