In a medium bowl, stir together condensed milk, vanilla, and Bourbon, if desired. In a large bowl, using and electric mixer, beat cream on high until stiff peaks form, 3 minutes. With a rubber spatula, gently fold whipped cream into condensed milk mixture. Pour into a 4.5×8.5-inch loaf pan. Freeze until firm, 6 hours (or covered, up to 1 week).