In a medium sauce pan combine butter and sugar and stir over medium heat until melted. Stir in corn syrup and condensed milk.
Bring to a boil and then decrease to simmer 7-10 minutes or until mixture achieves deep golden color, stirring constantly. (*For lower altitudes, simmer time may need to be reduced 2-3 minutes, watch carefully for coloring!) - See update below recipe!
Pour caramel into a foil-lined 8x8 inch pan and allow to cool completely. Sprinkle with coarse sea salt if desired.
When completely cooled, cut into squares and wrap in wax paper.