Preheat oven to 350 ℉ and line a half sheet pan (11×17'') with parchment paper or grease it.
In a small saucepan add 1 cup salted butter, 1 cup waterand ⅓ cup cocoa powderand bring to a boil. This blooms the cocoa making the flavor pop. While mixture comes to a boil prep the dry ingredients.
In a large bowl, whisk together the 2 cups sugar, 2 cups all-purpose flour, 1 teaspoon baking soda and ½ teaspoon salt. When cocoa mixture comes to a boil, quickly add it to the dry ingredients and stir to combine.
Add the 2 large eggs, ½ cup sour cream, and 1 teaspoon vanilla extract. Mix well.
Pour batter into prepared pan. Bake for 18-22 minutes, until the top is set. When cake has baked halfway, start the frosting.
FOR THE FROSTING: In a small saucepan, add the ½ cup salted butter, ⅓ cup cocoa powder, and ⅓ cup milk to the same small saucepan and bring to a boil over medium heat. Reduce heat and keep warm. Right when cake has been removed from the oven, remove the cocoa mixture from the heat and pour into a large mixing bowl with 1 teaspoon vanilla extractand 3 ¾ cups powdered sugar. Mix with an electric mixer to ensure there are no lumps.
Quickly pour warm frosting over warm cake and use a spatula to gently spread frosting evenly over the cake being careful not to tear the top of the cake. Do this quickly as the frosting will set up almost immediately.
Let cake cool to room temperature. Cut into slices and serve with vanilla ice cream. Store at room temperature for up to 3 days, chill in the fridge up to five days, or freeze up to three months.