Preheat oven to 350˚F. Line two baking sheets with parchment paper or silicon baking mats. (This is essential in order to remove the thin brittle from the pans.)
Beat butter and sugar until light and fluffy. Add corn syrup, vanilla and salt; blend thoroughly. Add eggs one at a time, beating well after each addition. Stir in melted chocolate. Fold in flour.
Divide dough evenly between the 2 prepared pans. Thinly and evenly spread batter into a 15 x10-inch rectangle on each pan. Bake for 25 to 30 minutes, switching between racks and rotating pans at least once, until edges are brown and brownies look dry. Watch carefully the last few minutes as the brownie brittle can overbake quickly. Remove pans from oven. Transfer bark on parchment from pans and cool on wire rack for 20 minutes. Break into pieces.
Recipe Note: For evenly shaped brittle, remove from pan and let cool 1 minute. Use a sharp knife to score into 2-inch pieces. Cool completely and then break into pieces.