King Arthur's Favorite Fudge Cake

Ingredients

• 2 large eggs at room temperature
• 1 teaspoon King Arthur Pure Vanilla Extract
• 1 cup water hot
• 2 cups King Arthur Unbleached All-Purpose Flour
• 1/2 cup unsweetened cocoa Dutch-process or natural
• 1/2 to 3/4 teaspoon salt to taste
• 2 cups granulated sugar
• 1/2 cup buttermilk or yogurt at room temperature
• 1 teaspoon baking soda
• 6 tablespoons buttermilk or yogurt
• 4 cups confectioners' sugar
• 8 tablespoons unsalted butter
• 16 tablespoons unsalted butter cold
• 1/4 cup unsweetened cocoa Dutch-process or natural