In a large glass mixing bowl combine butter, sugars and corn syrup. Microwave for 5 minutes. With hot pads, remove the bowl from the microwave and stir. Pour in one can of sweetened condensed milk and stir.
Place bowl back in the microwave and cook for another 10-15 minutes stopping the microwave and stirring every 3 minutes.
Check the consistency of the caramel by dropping a few drops into a bowl of ice water. If the caramel forms and soft ball, it is ready to be poured into the prepared pan.
Once the caramel is finished, carefully pour into greased pan and cover with plastic wrap. Place the caramel in a fridge overnight, or in the freezer for one hour, until cool enough to handle.
Remove caramel from the pan and slice into bite sized pieces. These can be wrapped in parchment paper for sharing.