Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 baking dish.
Combine the melted butter, sugar, and vanilla in a large bowl. Beat in the eggs, one at a time, mixing well after each, until thoroughly blended.
Sift the flour, cocoa powder, and salt in a bowl. Gradually stir flour mixture into the egg mixture until blended. Stir in the chocolate morsels. Spread the batter evenly into the prepared baking dish.
Bake in preheated oven until an inserted toothpick comes out clean, 35 to 40 minutes. Remove, and cool pan on wire rack before cutting.