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I'd mix up one egg and use half of it and two whole eggs. You could measure half with a kitchen scale or small measuring cup. The yolk and whites have different baking properties, so separating them might do things to the brownies you don't want. :)
I didn't know that (about the different properties)! Thanks! I will use my scale to measure.
Depends on whether you want dense or cakey brownies...do 2 eggs for dense, 3 for cakey. Good luck!