Preheat oven to 350 degrees. In a medium saucepan over low heat, melt butter and chocolate chips. Stir constantly and remove from heat when chips are fully melted. In the same pot, blend in the sugar and vanilla. Then add eggs, blending fully one at a time. Add flour and salt and beat until air bubbles form in the batter.
Pour into a greased 9 X 13-inch pan (this recipe works great doubled and poured into a large 11 X 17-inch rimmed baking sheet).
Bake for 20 minutes. Turn heat down to 325 degrees and bake for 10 more minutes.