Weigh or lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, cocoa, and salt in a medium bowl.
Combine butter, oil, and 3 ounces chocolate in a large bowl over simmering water. Cook over low heat until smooth, stirring occasionally. Remove from heat; cool slightly. Add beer, granulated sugar, brown sugar, vanilla, egg, and egg yolk. Beat with a mixer at medium speed 1 minute or until combined. Add flour mixture to chocolate mixture, stirring just until combined. Finely chop remaining 1 1/2 ounces chocolate. Stir chocolate into chocolate mixture.
Pour the batter into an 8-inch square glass or metal baking pan coated with cooking spray. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out with moist crumbs clinging. Cool in pan on a wire rack.