Microwave on High 7 to 8 minutes; stir well after 4 minutes.
At the end of 8 minutes, add butter and vanilla. Blend well. Return to microwave and cook on high for 1 or 2 minutes more.
Remove and add baking soda. Stir until light and foamy. Pour immediately on greased cookie sheet. Cool 30 to 60 minutes. Break into pieces and store in airtight container.