In a large mixing bowl, whisk together the flour, sugar, baking soda, salt and keep aside.
Place a nonstick saucepan over medium heat and add butter, water, cocoa powder.
Mix and bring to a boil. There should be no lumps and mixture should be smooth.
Add this chocolate mixture in the bowl of dry ingredients and mix until partially combined.
Add buttermilk, eggs, vanilla extract and mix until just combined.
Pour batter in a greased and floured half sheet pan (Dimensions: 18x13) and spread it evenly with a spatula.
Bake at 350 degrees F for about 20 minutes or until an inserted toothpick in the center comes out clean.
When the cake is halfway through baking, prepare the frosting/ glaze.
Place a nonstick saucepan over medium heat and add milk, butter, cocoa powder.
Mix and bring to a simmer. The mixture should be smooth.
Remove from heat and mix in vanilla extract, powdered sugar until glaze is smooth.
Stir in chopped pecans, if you like. I didn't add them.
Remove cake from pan and let it cool for 5 minutes.
Then, pour the hot icing on on top of the warm cake and spread it evenly with a spatula.
Allow the frosting to set for 20-30 and then cut slices and enjoy. It will be slightly warm and ooey gooey. If you prefer your icing to set firmer, let the frosted cake continue cooling further at room temperature for 1-2 hours or you can chill in the fridge for 45 minutes- 1 hour.