Preheat the oven to 350°F. Spray a 9x13-inch baking dish with nonstick spray
MAKE THE CAKE:
In a large bowl, whisk together the flour, sugar and baking soda. Set aside.
In separate bowl, combine the buttermilk, egg and vanilla and whisk together. Set aside.
In a medium saucepan, melt the butter over medium heat. Stir in the cocoa and water and bring the mixture to a boil. Pour the boiling mixture into the bowl with flour mixture and whisk together well. Stir in the buttermilk mixture. When all is well blended, pour the batter into the prepared baking dish.
Bake 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.
PREPARE THE GLAZE:
Melt the butter in a heavy saucepan over medium-low heat. Add the remaining ingredients, powdered sugar ½ cup at a time. Pour the warm glaze over the cake as soon as you remove it from the heat.
Notes
*It's very important to sift the powdered sugar to avoid lumps.
*This is so rich and delicious that you may want to plan serving it with a nice scoop of good vanilla ice cream.