How to enhance sweetness of a baked chocolate brownie?

Serve it with whipped cream or ice cream.

Next time, I imagine you'd have to increase the amount of sugar you use.

You could always frost the brownies, or you could also sprinkle powdered sugar with a sieve over the top to add a little extra sweetness.

Obviously, (as mentioned before), you could add a little bit more sugar, or you could add instant cocoa which is essentially cocoa powder with a bit of dried milk and sugar added. This would work, though the full flavour of the cocoa may disappear a bit.

If it were me, I would dump the idea that these were eat-alone brownies, and see what else could be done with them. That bitter chocolate makes me think of Mexico. How about this: Oaxacan turtle sundaes, with vanilla ice cream, your brownies, caramel sauce, and ancho-dusted pecans (ancho is a dark red pepper with a fair amount of heat. Ground ancho can be found many places with a decent spice selection or Mexican grocery). For extra points, sub cajeta (caramelized goats milk) for the caramel sauce

Another way would be to slice the brownie down to very thin slices first, and then frost it as suggested by @Katherine Pitta. The combination of bitterness and sweetness will make it a very nice companion with tea or coffee.

I would crumble it all up on a mixing bowl then add some honey or goldrn syrup. I would the mix it. Then I mould them back then freeze them.

U could always crumble the brownies and add frosting, and make cake pops or “brownie pops” it’s super easy crumble the brownie mix in frosting the mold into a ball, poke a stick into it and dip into any coating (candy melts, icing, even frosting) add sprinkles on top for more sweetness, freeze and enjoy! ??

I had the same problem a couple of days ago, I ate a slice of brownie slightly warmed up with some vanilla ice cream and melted some milk chocolate on top. It was delicious, and perfectly balanced the bitterness of my brownie.

It would also work if you turned the brownies into cake pops!

Pour/spread over them sweeter chocolate melted and sprinkle lightly with high quality flaky salt like Maldon. The chocolate is only for the purpose of the salt sticking to it. I mean literally 5-7 crystals per brownie square. This will work in ratio for brownie recipe 1:1:1:1:1 butter, chocolate,eggs,flour,sugar by weight. Next time I would use dark cocoa (nixtamalized), like the one which goes into Oreo. If you need to have more cocoa in it, the best I thing would be just very dark chocolate 70%+ at the start, without the cocoa. Chocolate has better balance in solids flavour.