Add remaining liquid ingredients and mix on low speed with an electric mixer or by hand with a spatula.
Lightly grease the bottom and sides of a 9x13 glass baking tray with vegetable oil. Next, lightly sprinkle and rub some flour on top of the greased tray. This will help keep the brownies from sticking to the glass tray after baking.
Bake for the time indicated on the package or until brownies are done.
Let them cool completely. Sprinkle powdered sugar on top and serve.