This pretty little egg cup was made by Joseph Machin & Co. of Nile Street. Burslem, Staffordshire. Although not marked, it is part of a breakfast set with a matching plate, which is marked with pattern number 862. The pattern (and corresponding pattern number) is shown on page 139 in the book, "Machin Porcelains and Earthenwares", by Bill Thom and Philip Miller.
The cup is 2-1/8" high. It is in excellent condition, and rings when tapped, despite a firing flaw (in-the-making) at the bottom of the cup's interior. There are no chips, no cracks, and no repairs.
In the early 19th century Joseph Machin and Company were one of the principal manufacturers of china and earthenware rivaling makers such as Wedgwood, Spode and Minton. However, due to the bankruptcy of the Stoke-based factory in the late 1830s the name disappeared from public knowledge. Their wares have always been popular with collectors but were often wrongly cataloged as being by those makes whose names have survived.
Note: The matching plate and coffee cup are available here in my shop.