Preheat oven to 350°.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, cocoa, baking soda, and salt in a small bowl, stirring with a whisk.
Place the chocolate chips and the butter in a large microwave-safe bowl. Microwave at HIGH 1 1/2 minutes or until the chocolate chips and butter melt, stirring every 30 seconds. Cool slightly. Add sugar and next 3 ingredients (through vanilla extract), stirring well with a whisk. Microwave at HIGH 1 minute or until sugar dissolves, stirring every 30 seconds. Fold in the flour mixture, stirring just until moist. Spread batter in a thin layer into a 9-inch square baking pan coated with cooking spray. Bake at 350° for 20 minutes or until a wooden pick inserted in center comes out almost clean. Cool on a wire rack.