This is a vintage reverse glass painting, also known as a silhouette foil print

This type of artwork is known as reverse painted foil art or tinsel painting. It is a vintage technique, often seen in pieces from the Art Deco era, that features an image painted on the back of glass with metallic foil or glitter applied behind it to create a shimmering, reflective effect. The style often depicts romantic or courting scenes with couples dressed in period attire, such as those from the 18th century. Tinsel painting was a popular decorative art form in America during the mid-19th century and was also a widely practiced folk art in parts of Europe.