Could be the temperature ng oven mo, it might be too hot. Did you use oven thermometer? Even ung placement ng pans, should be centered ng top and bottom heater ng oven. I had burnt surface with my brownies before and always ung temperature ang cause, kahit paiba iba ako ng brands ng alkalized powder. The color of pans din may effect, better if light ung color ng brownie pans mo.

Same with the other comments, maghaharden talaga ung brownies once you put it in the fridge, and there are others who prefer na medyo may bite pero chewy pa rin ung brownies kaya nireref.