Preheat your oven to 350°F (175°C) and grease a large baking sheet.
In a saucepan, combine water, butter, and cocoa powder, and bring to a boil.
In a large bowl, mix sugar, flour, and baking soda.
Add the cocoa mixture to the dry ingredients and stir well.
In another bowl, whisk together buttermilk, eggs, and vanilla.
Combine wet and dry ingredients until smooth.
Pour the batter into the prepared baking sheet.
Bake for 20 minutes or until a toothpick comes out clean.
Let the cake cool before frosting it with chocolate frosting.
Serve and enjoy your delicious Texas sheet cake.