Preheat your oven to 350 degrees F. Spray a 9-inch round baking dish or pie plate with non-stick spray. Set aside.
In a large mixing bowl, whisk together the flour, cocoa powder, sugar, brown sugar, baking powder, and salt.
Add the buttermilk, oil, and vanilla to the dry ingredients. Stir together until well combined with no dry patches.
Spread the batter in the bottom of the baking dish and smooth it out in an even layer.
In a medium bowl, stir together the sugar, brown sugar, cocoa powder, and chocolate chips. Sprinkle this mixture evenly over the cake batter. Pour the boiling water over the top of the cake, 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.
Remove the cake from the oven and let it cool on a wire rack for 15 minutes before serving.
Notes
If you don't have buttermilk on hand, you can use sour milk instead. Pour 2 teaspoons of white vinegar or lemon juice into a measuring cup and then fill to the 3/4 cup mark with milk. Stir that together and let it sit for 5 minutes before using.
You can leave the mini chocolate chips out of the sauce if desired. I just like the added chocolate decadence they give this cake.
You want to make sure you don’t overbake this cake. It will still be a little jiggly in the very center and the sauce will be bubbly up through some holes in the cake when you take it out of the oven. Don’t be alarmed, this is normal. The longer the cake sits the more it will set up, so serve it while still warm.
Store any leftovers in the fridge for up to 5 days. Reheat in the oven or microwave until just warm. You can freeze this cake for up to 3 months.