Preheat the oven to 350°F. Spray a 9-inch baking dish with baking spray, set aside.
In a large bowl whisk together the flour, cocoa powder, sugar, brown sugar, baking powder, and salt.
Add the buttermilk, oil, and vanilla.
Stir together until combined with no dry patches.
Add the batter to the baking dish and smooth it out in the bottom of the dish.
In a separate bowl make the sauce by stirring together the sugar, brown sugar, cocoa powder, and chocolate chips.
Sprinkle this mixture over the batter.
Pour the boiling water over the top, do not mix it in. Bake for 28-32 minutes until the top looks set and cracked and the sides and starting to pull away from the dish.
Let the cake cool on a wire rack for 15 minutes before serving.
Notes
The chocolate chips are optional in this cake.
This is a really rich chocolate cake, so perfect for chocolate lovers.
Let sit 15 minutes to set up.
This can be frozen, see tips above on how to do that.
Other extracts can be added to change the flavor, some ideas are included above.
Top with some of your favorite toppings, see above for suggestions.