Cut the chocolate in small pieces and cook with the brown and granulated sugar, the condensed milk and the corn syrup. Stir until the sugar is dissolved. Then cook, stirring occasionally, until the fudge forms a soft ball when dropped in cold water. Remove from the fire, add the butter and let stand until it is cool. Then beat until it begins to lose its glossy appearance. Add the vanilla and the nuts, pour on a greased pan and allow to cool. Cut in squares.