The pan size matters when the temperature in which you have to bake is taken into consideration. If the pan makes the batter shallower than the requirement, raise the temperature and decrease the baking timings. And if the pan makes the batter deeper than the requirement, decrease the temperature and increase the baking timings.
Ingredients which you use to bake should be at room temperature. Because if it is stored in a cold temperature and used directly, it releases vapor which like acid to your cake.
Inserting a toothpick is recommended because toothpicks are thin and it causes little holes in your brownie rather than using knives which would causes big holes, destroying your icing.
This is a basic learner’s rule. You shouldn’t cut the brownie when it is hot or warm. Wait for it to cool because if cut when hot, it will crumble into pieces and you wouldn’t want. You want whole square pieces.
Again a basic rule but even if you are using a parchment sheet or butter paper, try greasing it first and then put the parchment sheet to be on the safe side.
FUN EXTRA TIP – sprinkle cocoa powder on the bottom of the pan or on top of the parchment sheet, restricting your brownies to turn white from beneath.