Preheat oven to 350° F. Butter a 9- by 13-inch baking pan.

Place the butter and baking chocolate in a small sauce pot. Cook the mixture over medium heat, stirring occasionally, until the butter melt, about two minutes. Remove from the heat and continue stirring till all the chocolate is melted. Add the sugar and fudge sauce to the pot and mix in. Whisk in the eggs and vanilla, then fold in the flour, salt, and chocolate chips until incorporated.

Pour the batter into the prepared pan and bake for about 30 minutes. The top should be crackly and a tester inserted into the middle should come out clean. Remove from the oven and let cool in the pan.