Preheat Oven: Preheat your oven to 350°F (175°C) and grease an 8-inch square or round baking pan.
Mix Dry Ingredients: In a large mixing bowl, combine the flour, sugar, cocoa powder, baking soda, baking powder, and salt. Whisk until well combined.
Add Wet Ingredients: Add the egg, vegetable oil, buttermilk, and hot water to the dry ingredients. Whisk until the batter is smooth and well combined.
Bake the Cake: Pour the batter into the prepared baking pan. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.
Prepare the Frosting: In a saucepan, melt the butter over medium heat. Stir in the cocoa powder until smooth. Remove from heat and whisk in the powdered sugar, milk, and vanilla until the frosting is smooth and spreadable.
Frost the Cake: Once the cake is completely cool, spread the frosting over the top.
Serve: Slice and enjoy your delicious one bowl chocolate cake!
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