Maddison scored one goal and made another as the Foxes came from a goal down to win 3-1 at the Brentford Community Stadium on Sunday.
The midfielder last played against Brentford when scoring the winner for Norwich City in a Championship clash in 2018.
Frank was an assistant coach on Dean Smith’s staff at the time of that fixture and says he has been impressed by the way the Englishman has flourished since.
“What a player,” Frank said. “His development has been fantastic. The assist he makes for the goal, but also his touch and decision making, the way he finds space between the lines is incredible.
“And also his work-rate, even in a game where they’re leading 3-1, in the end he just keeps going with pressure and counter-pressing. That’s what makes a top player.”
Frank made six changes to his side for Sunday’s fourth round tie and Brentford’s exit leaves them free to focus on a tilt at automatic promotion.
The Bees are unbeaten in 16 in the Championship but two Covid postponements have left them with a particularly gruelling schedule over the next two months and Frank admitted prioritising the league run came into his thinking after Leicester had turned the tie around in the second half.
He added: “We want to win every game and we put a team out there that was strong enough to compete, especially in the first-half, similar to what we played against Southampton and West Brom [in the Carabao Cup] and those teams.
“But I understand that the substitutions I made, I would have done differently if we were still leading one-nil. If we were leading one-nil then it’s always good to continue that winning mentality and good performance and feeling.”