Preheat the oven to 350 degrees F. Grease a 9 x 13-inch baking pan and set aside.
In a large mixing bowl, combine flour, 1 cup of the brown sugar, baking powder, baking soda, and salt. Whisk until well combined. If there are lumps of brown sugar, use your hands to break them up. Stir in chocolate chips and set aside.
In a small saucepan, melt the butter and chocolate together over medium-low heat.
In a separate small saucepan, heat buttermilk over low heat until barely warmed. You don't want it to bubble or boil. Remove from heat.
Remove chocolate and butter mixture from heat and stir in the vanilla. Pour mixture over dry ingredients. Add the buttermilk and stir until combined. Spread into the prepared pan.
Combine the remaining 1 1/4 cups brown sugar with the cocoa in a small bowl. Whisk until smooth, using your hands to break up any brown sugar clumps. Sprinkle mixture evenly over the chocolate cake batter.
Pour the boiling water evenly over the cake. Yes, it will look like a mess, but I promise it will turn out!
Carefully transfer the pan to the oven. Bake for 30-35 minutes or until the center is firm to the touch.
Remove cake from oven and cool on a cooling rack for at least 30 minutes before serving. To serve, invert each serving on a plate so that the fudge sauce on the bottom becomes a topping. Spoon any extra sauce in the pan over the top. You can serve the cake at room temperature or warm. Top with ice cream, if desired.