Combine sugar, butter, and Karo syrup in a small saucepan. Cook over medium heat until thickened. Pour syrup mixture into 9-x13-inch pan. Place 6 slices of bread on top of syrup mixture. Top with remaining 6 slices of bread. Combine eggs, milk, vanilla, and salt, stirring until blended. Pour over bread slices. Cover and chill overnight. Bake uncovered at 350 degrees for 40–45 minutes.