What song is Kyoko dancing to in Puella Magi / Mahou Shoujo Madoka Magica?
It's part of the song Connect. You can say it's a remix version. The title is Connect -Game Installation-

It's from Mahou Shoujo Madoka Magica Special CD 6 (track #13).

Mahou Shoujo Madoka Magica has total 6 Bonus CD. From what I could see, the artist is still ClariS. (I actually has all the bonus CD)