Make the Brownies:

1. Preheat the oven to 350°F. Grease a 9-inch square baking pan and line it with parchment paper so that it hangs over two sides to make it easy to life the brownies out of the pan when baked.
2. Melt the chocolate with butter in a medium heatproof bowl over a pan of simmering water. Make sure the bottom of the bowl does not touch the water, or you will burn the chocolate. Whisk until melted and smooth. Remove from heat.
3. Add sugars into the chocolate mixture and whisk until completely combined.  Then, whisk in eggs and vanilla, until just combined. 
4. Add in flour, cocoa powder, and salt and fold together using a large spatula until just combined. Transfer the thick batter into the prepared baking pan.
5. Bake for 25-30 minutes, until a toothpick inserted comes out clean with moist a few moist crumbs attached. If it comes out clean, it's overbaked.

Make the Caramel Sauce:

1. Meanwhile, make the caramel sauce by heating sugar in a large shallow saucepan over medium-high heat. Whisk continuously as the sugar as it begins to melt. It will form clumps but will eventually melt completely. This can take 10-15 minutes. Continue to stir continuously until the sugar reaches a deep amber colour. Keep a close eye because the sugar will burn if you overcook. 
2. Add the butter (note that the butter will immediately begin bubbling and may splatter, so be careful). Whisk until the butter is completely melted, then cook for 1 minute without stirring.
3. Slowly pour in the heavy cream and whisk until combined in the mixture. Again, be careful as the mixture will bubble and may splatter. Remove from heat and allow the sauce to cool in the saucepan for 10 minutes. The caramel will thicken as it cools. 

Add the Topping:

1. Once the brownies are done, remove the baking pan from the oven and place it on a wire cooling rack. Allow the brownies to cool for 10 minutes in the pan, then remove from the pan and place on the wire cooling rack.
2. Pour the slightly cooled caramel sauce on top of the brownies. Spread it evenly with a metal spatula.
3. Allow the caramel to set, then cut into squares. You can place it in the refrigerator to help it firm up faster.