In a large bowl, combine softened butter, brown sugar, and white sugar. Mix with an electric mixer for about 3 minutes on medium/high speed until light and fluffy.
1 cup salted butter, 1 cup brown sugar, 1 cup granulated white sugar
Add in the vanilla and eggs one at a time and mix well in between, until the sugar starts to dissolve and the mixture is fluffy, about 1-2 minutes.
1 teaspoon vanilla extract, 2 large eggs
In a separate bowl, combine the all-purpose flour, cocoa powder, salt, and baking soda.
3 cups all purpose flour, ¾ cup cocoa powder, 1 teaspoon baking soda, ½ teaspoon salt
Add the dry ingredient mixture to the wet ingredient mixture in batches, mixing well in between. Mix for a good 1-2 minutes, so that the dough is mixed well. The texture will be similar to sticky chocolate cookie dough.
Save one cup of dough and set it aside. Press the remaining brownie dough evenly into the bottom of a 9x13 inch baking pan.