Preheat oven to 350F (glass 325F). Butter and flour a 9 X 13-inch pan. and set aside.
In a medium-size saucepan, melt the unsweetened chocolate with the butter over medium heat. Set aside to cool until barely warm.
In a large bowl, beat eggs, sugar and vanilla until light-colored and well blended. By hand, stir in the chocolate mixture and blend. Add flour and mix until blended. Do not overbeat(must be by hand).
Pour into prepared pan. Sprinkle chocolate chips evenly on top, and carefully run a knife through the batter to immerse some chips.
Bake for 35 minutes, or until a knife inserted in the center comes out clean. Do not over cook. Cool om a rack completely (at least 3 hour before cutting. Store in a covered tin.
To freeze, place individually wrapped brownies in a plastic bag and freeze up to 1 month.