Preheat your oven to 350 degrees Fahrenheit.
Grease a 13x18 inch sheet pan to keep the cake from sticking.
In a large mixing bowl, combine flour, sugar, and salt.
Set this mixture aside.
Melt the butter in a saucepan over medium heat.
Stir in cocoa powder and boiling water until mixed well.
Pour this hot mixture over the dry flour mixture.
Stir lightly to combine, letting it cool slightly.
Mix together buttermilk, eggs, baking soda, and vanilla extract in a separate small bowl.
Add to the butter-flour mixture once cooled.
Stir until smooth and fully combined.
Pour the batter into the prepared sheet pan and spread it evenly.
Bake for 20 minutes or until a toothpick inserted comes out clean.
Let it cool before serving.