1 can condensed milk (not evaporated!)
2 lbs (5 1/3 cups) light brown sugar
1 pint (16 oz) light Karo syrup
3/4 lb butter (1 1/2 sticks)
One pinch of salt
2 tsp vanilla
Put everything EXCEPT vanilla into a large saucepan or pot.
Cook, stirring occasionally (carefully) until it reaches "Hard Ball Stage" which is 120-126C. (About 250 F.) on candy thermometer.
When it reaches the right temp, add vanilla & pour into buttered 10 x 14 cookie pan.
Cool overnight or until firm.
Loosen around edges and invert onto counter top. Cut into 1" blocks and wrap in wax paper.