In a 9x13 inch dish, generously butter a 9x13 inch pan and set aside.
In a microwave safe bowl (make sure it is microwave safe!) melt the butter. Once the butter is melted add brown sugar, karo syrup, and sweetened condensed milk and mix well. Microwave on full power for 4-5 minutes.
Remove from microwave and stir. Continue microwaving for 4-5 minutes. Stir again and add the vanilla.
Pour the caramel into the 9x13 inch buttered pan. Let them completely cool for 1-2 hours. Cut the caramel with a knife and wrap in wax paper. Keep refrigerated.