Bring the milk, corn syrup, and sugar to a boil under medium heat. Add the cocoa powder, butter, salt, and vinegar. Continue to boil with constant stirring until the firm-ball stage (or until candy thermometer reaches 248°F). Pour into a buttered pan and let cool until it can be handled without burning. Use buttered hands to work (fold over and pull -- using only finger tips -- again and again) the fudge until it is hard to pull. Pull into a twisted rope about 1/2" thick and cut into 1" sections. Let cool on waxed paper. Makes about 100 sections.