Preheat the oven to 350 degrees F (175 degrees C). Line two 12-cup muffin tins with paper liners.
Combine water, butter, oil, and chocolate in a large saucepan. Heat over medium heat until melted and fully combined.
Whisk flour, sugar, cocoa, baking soda, and salt together in a large bowl. Pour in hot chocolate mixture and whisk until combined. Whisk in eggs, one at a time. Whisk in buttermilk and vanilla extract.
Spoon batter into the prepared muffin cups, filling each 2/3 full.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 17 to 22 minutes. Cool for 10 minutes.