Preheat oven at 250 degrees F. Prepare a 9" X 9" baking pan with aluminum foil allowing foil to hang over the edges and spray with cooking spray.
Bring stout to a boil and reduce to simmer and cook for about 10 minutes or until reduced to 1/4 cup. Set aside to cool.
In a double boiler or a small bowl atop a small saucepan filled with 1 inch of water, melt chocolate chips and butter over high heat, stirring occasionally, until melted.
Pour melted chocolate into a medium bowl and cool slightly. Add sugar, vanilla, salt and stout and mix well with a whisk or wooden spoon. Add eggs one at a time, mixing just until combined before adding the next egg.
Fold flour into mixture and pour into baking pan. Bake for one hour or until a toothpick comes out clean. Allow to cool in pan for 2 hours or overnight to firm up for best cutting. Dust with powdered sugar if desired.