Preheat oven to 350℉. Spray a 9″x13″ pan with non stick spray and set aside.
In a small bowl combine 1 cup brown sugar and cocoa powder, mixing until there are no lumps. Set aside.
In a large bowl whisk together flour, 1 cup brown sugar, baking powder, baking soda, espresso powder and salt. Set aside.
In a medium size sauce pan over medium heat melt unsweetened chocolate with the butter, stirring until smooth. Meanwhile warm buttermilk on the stove over medium heat or in the microwave until just warm. Whisk buttermilk into melted chocolate and add vanilla. Add liquid mixture to the dry ingredients all at once, whisking until well combined and smooth. Stir in chocolate chips. Spread evenly into prepared pan.
Sprinkle cocoa powder and brown sugar mix evenly over the entire top of cake batter. Immediately pour the boiling water over the entire surface DO NOT STIR. Bake for 30-35 minutes. Allow to cool for at least 15 minutes prior to serving. Scoop out and invert onto serving dish so the chocolate sauce tops the cake. Can be served warm or at room temperature.
Place a large baking sheet on the rack below the cake pan to catch any drips that overflow.