Mix together the melted butter, sugar, brown sugar, corn syrup, sweetened condensed milk, and salt in a large, microwave-safe bowl.
Microwave on high for 2 minutes 30 seconds. Remove and stir. (Time based on 1000 watt Microwave. You may need to adjust power and time for your microwave wattage.)
Microwave for another 2 minutes 30 seconds. Remove and stir.
Microwave for an additional 2 minutes. Remove and stir in the vanilla.
Pour the caramel into a generously greased 9x9 dish and refrigerate for 20 minutes. Let it sit at room temperature until it is firm enough to cut into pieces.