Preheat oven to 350 degrees F. Grease a 13x18 inch sheet pan that has a lip at least 3/4" high around it.
In a large mixing bowl, stir together the flour, white sugar, baking soda and salt. Add sour cream, eggs and vanilla to the ingredients. Using an electric mixer to combine ingredients, but do not over-mix.
In a medium saucepan, over medium heat, melt butter and cocoa powder together and whisk together.
Stir the instant coffee into the boiling water. Pour instant coffee and water mixture, and brown sugar, into the saucepan with other ingredients. Whisk until smooth while bringing mixture to a boil. Once a low boil starts, remove from heat and sit aside to allow it to cool slightly, approximately 10 minutes.
Gradually add the chocolate, butter and coffee mixture to the batter in the large mixing bowl, stirring constantly. Mix until smooth and free of any lumps.
Pour batter into prepared baking pan and smooth to evenly distribute into the pan. Bake for 15 minutes. Test by poking a toothpick into the middle of the cake. If the batter is still very wet, continue baking for another 5 minutes. Then remove it from the oven. It is okay if the toothpick still comes out a little dirty. We don’t want to over-bake the cake.