You can use either type of fat, but butter tends to make a more cakey brownie and oil leads to a more fudgy brownie. Both are chocolate, both are good.
If you want fudgy brownies, use a higher fat-to-flour ratio, which can be butter, oil and/or chocolate. These moist brownies won’t rise as high, but can still be delicious.
Cake-like brownies use more flour than fats and tend to rise higher being light and fluffy.
Vanilla extract certainly adds to the flavor of brownies, but chocolate is really the star of the show, so vanilla can be omitted if desired.
But you can also use other flavorings and extracts instead of vanilla. My favorites are almond, rum and butter.
Try adding 1-2 teaspoons of extract to your box cake mix too. It is such a small amount of liquid that you don’t need to worry about adding more dry ingredients.
A cookie bar is just batter or dough spread into a baking dish and baked into a large bar. Sometimes they are more like brownies, other times cookies and sometimes a little of both, these are called brookies.
Cut cookie bars into squares or diamonds for a little bit of pizazz.