Preheat oven to 350℉. Lightly grease and flour a 9x13-inch cake pan.
In a saucepan over medium-low heat combine butter, water and cocoa powder. Whisk frequently until the butter is melted and the mixture is smooth and combined.
In a large mixing bowl, combine flour, sugar, and salt.
In a separate bowl, whisk together eggs, buttermilk, baking soda and vanilla.
Add slightly cooled butter mixture and egg mixture to dry ingredients. Stir until mixed.
Pour cake batter into prepared 9x13-inch baking pan. Bake at 350℉ for 30-40 minutes.
Chocolate Glaze
Melt butter in saucepan. Stir in cocoa powder, 3 Tbsp. half and half, powdered sugar and vanilla. Stir until chocolate glaze forms. Add additional Tbsp. of half and half if glaze is too thick. Pour warm frosting over slightly cooled cake. Serve cake once frosting sets.
Notes
Baking time: In my oven, the cake typically takes 35 minutes. Do not over bake.
Buttermilk: I use full-fat HEB Buttermilk. Be sure to shake well before pouring into measuring cup.
If your flour is lumpy, sift before mixing. I like to sift my flour before adding it to bowl, but it is optional.