In a large sauce pan, add butter and chocolate and heat over low heat, stirring constantly. Continue heating and stirring until completely melted and smooth.
Remove from heat and add sugar. Stir to combine. Allow mixture to cool slightly.
Beat in the eggs, one at a time, mixing well after each addition.
Add vanilla and stir to combine.
Add flour and salt into the chocolate mixture, and stir to combine.
Pour batter into a greased 9"x13" baking sheet.
Bake in a 350° oven for 30-35 minutes, until cooked through and a toothpick inserted comes out clean with just a few fine crumbs.