Preheat your oven to 350° F.
In the microwave, melt the butter in a medium sized microwave safe bowl (about 40 seconds).
Add the granulated sugar and brown sugar to the melted butter, stir to combine.
Once the sugars are added in, microwave the mixture for an additional 1 minute.
Using an electric hand mixer, add in the vanilla extract and 1/2 cup of the flour to the bowl and mix constantly.
Add in the egg and continue mixing until completely combined.
Next, add in the remaining 1/2 cup of flour, cocoa powder and salt. Mix until well combined.
Finally, fold in your desired mix-ins (chopped chocolate, chocolate chunks, chocolate chips or nuts).
Pour into a greased 9x4 bread pan.
Bake at 350° F for 17-23 minutes or until a toothpick comes out with just a few moist crumbs on it.
Allow the brownies to cool in the pan before cutting and enjoying.