Begin by cubing your butter and adding it to a saucepan with unsweetened cocoa powder and water. Place the saucepan over medium heat, stirring continuously until melted, combined, and smooth—about 5 minutes.

In a large mixing bowl, whisk together the buttermilk, eggs, baking soda, and vanilla extract until fully combined.

Pour the cooled butter and cocoa mixture into the buttermilk mixture, gently whisking together until smooth.

Gradually add sugar, flour, and salt into the wet mixture, stirring gently with a spatula until no dry flour remains.

Preheat your oven to 350°F (175°C) and pour the batter into a greased 9"x13" baking pan. Bake for 30-35 minutes or until a toothpick inserted comes out clean.

While the cake cools, melt butter in a saucepan over low heat, stir in buttermilk and cocoa until combined, then gradually add powdered sugar and vanilla until smooth.

Once cooled, pour the frosting over the cake, spreading evenly.