Preheat oven to 350°. Place chocolate in a large bowl. In a 10-in. cast-iron skillet or other ovenproof skillet, combine butter and stout. Bring to a boil; reduce heat. Simmer 10 minutes, stirring constantly. Pour over chocolate; stir with a whisk until smooth. Cool slightly. In another large bowl, beat egg, egg yolks and sugars until blended. Stir in chocolate mixture. In another bowl, mix flour, baking cocoa and salt; gradually add to chocolate mixture, mixing well.
Spread into skillet. Bake until set, 25-30 minutes. Cool completely in skillet on a wire rack. If desired, serve with vanilla ice cream.