United still carried the greater threat, even if enjoying less possession, with Sama's shove on Janzuaj deemed unworthy of a third penalty and a rapid break leading to Lingard forcing Ward to tip over the bar following good work by Lawrence.
So it was no surprise when Cole whistled in his hat-trick, with the aid of a slight deflection, after a scramble from a Januzaj corner, that seemed to include a Sama handball, before Thorpe laid invitingly back to Cole on the edge of the area.
Two minutes later and the visitors lost their heads with Teixeira booked for clattering into Januzaj and Flanagan harshly sent off for kicking the ball against the grounded Belgian. Things got even spicier for a spell and Januzaj picked up a booking himself but the fact was the Liverpool players struggled to handle him all evening.
Cole was unlucky not to add a fourth when his deflected shot looped off target and Lingard had yet another penalty shout turned down while the Merseysiders threatened from a couple of set-pieces but never really tested Ben Amos.
Under-18 trio Ben Pearson, who quickly displayed a fantastic touch, Liam Grimshaw and Andreas Pereira got a taste of the action as Warren Joyce's side closed out a fabulous win to set up another Old Trafford date with Tottenham in the final on Monday 20 May.
United: Amos; Vermijl, Thorpe, M.Keane, James; Cole (A.Pereira 84), Ekangamene, Tunnicliffe, Januzaj (Grimshaw 80), Lingard; Lawrence (Pearson 75). Subs: Sutherland, Wilson.