Millwall U21s 2 Cardiff City U21s 1
Millwall were much the better side in the opening exchanges, keeping possession well and forcing the League leaders to sit deep in their own half.
At the other end, Cardiff's first chance fell to Brad Williams, but the midfielder could only head Tyler Roche's cut-back over the bar.
The visitors then enjoyed a spell on top but Stephen Bywater kept his side in front with a smart stop from Kane Owen's volley.
Aiden O'Brien looked Millwall's biggest threat in attack, and his cross narrowly evaded the head of Josh Wright who had broken into the box.
Millwall doubled their advantage in the 67th minute when Connor Hunte played a neat one-two with Aiden O'Brien on the edge of the box, racing onto the return pass and slotting past keeper Ben Wilson.
Powell nearly made it 3-0 with five minutes to go, but his powerful free-kick from 30-yards was tipped over by Wilson before O'Sullivan smashed the ball home to halve the deficit two minutes into stoppage time.
After the game, Chopper was delighted not only with the result but a number of plus points from the performance. He said: "First of all I was pleased with the goals, especially the first one which was something we had worked on in training.
"I thought we kept the ball extremely well, particularly in the early stages, and having senior players in the side like Nicky Bailey, Josh Wright and Jack Smith clearly helped the youngsters in that respect.
It was good to see Nicky back in action after his injury lay-off and hopefully he will be a welcome addition to the senior squad again before too long."
Millwall: Bywater; Siafa, De Havilland, Wood, Smith; Powell, Bailey (Farrell, 71'), Wright; Hunte (Guinchard, 74'), Philpot, O'Brien (Parr, 84'). Subs not used: Peter, Goodman.