The first "secret" ingredient was brewed coffee – not in any specific amount, just a good splash from the coffee pot as she beat the eggs. (She did mention that the eggs were on the small side, so this extra liquid didn't seem to affect the consistency.) We've actually written about how espresso or dark coffee can enhance the flavor of chocolate. It boosts the richness without adding an overtly coffee flavor.
The second special ingredient was cinnamon. Cinnamon is a classic complement to chocolate (Mexican Hot Chocolate being my favorite example), yet I had never thought of adding it to brownies. Again, she didn't really measure – just a dash – but if you want a more specific recipe, you could try these Sweet Heat Brownies with cinnamon and chile.
Now it's your turn – what secret ingredients or techniques do you have for achieving brownie bliss?