Virgin and Child with the Young Saint John the Baptist by Sandro Botticelli and Workshop (1490) - Public Domain Catholic Painting

The perfectly round form, called a tondo, became popular during the 1440s. Made for domestic settings instead of churches, the circular format challenged the artist to create a harmonious, balanced composition within this more difficult shape. The attribution has been a matter of debate. Botticelli often collaborated with students, including Filippino Lippi, who would himself become a significant painter. Few specialists have doubted that Botticelli executed the central passages: the delicate modeling of the faces, the graceful poses, the figures’ profound interiority, and the diaphanous veil are the artist’s hallmarks. However, someone else in Botticelli’s studio probably painted Mary’s blue garment, also the most heavily restored part of the painting.

Source