Combine butter, water, and cocoa powder in a small saucepan. Let them melt using low heat. Bring to a simmer and stir constantly. This process doesn't take long, about three minutes.
Remove from heat and add cocoa mixture to flour mixture. Beat with an electric mixer slowly until combined. Use a spatula to scrape the mix stuck on the side of the bowl.
Add egg, buttermilk, and vanilla; beat on low speed for two more minutes or until no lumps. Let this mixture cool down before moving on to the next step.
Turn the batter into the prepared pans and bake until a toothpick inserted in the center will come out clean. I usually start checking 15 minutes from the start of the baking process.
Let cool in the pan for 10 minutes and on the rack before frosting.