At first glance, ramekins and custard cups appear to be the same, given how similar they are in size and shape. To the casual cook, the two might indeed be interchangeable for common uses. Either works well for individual portions of foods such as desserts and condiments. For certain specialty dishes, such as creme brulee and souffles, the shape of the dish becomes crucial.
Straight Sides for Rising
Ramekins are most easily identifiable for their straight sides, which are important for achieving the necessary rise in dishes like creme brulee. These small dishes round usually hold 6 ounces, though 4- and 8-ounce containers are available. Ceramic ramekins are most common, but they are also made from heatproof glass and stoneware. Many ramekins feature ridges on the outside and might have a horizontal band at the top, with the inside perfectly smooth and straight.
Ramekins for Souffles and Soups
Souffles must be made in ramekins or a similar deep dish with straight sides that encourage the souffle to rise upward as it bakes. These cups, particularly the larger 8-ounce size, also work well for individual dishes of French onion soup. Wider, shallow ramekins are typically used for creme brulee -- the shallow depth makes it easy to access the top with a torch. Dole out individual portions of pudding in ramekins, or bake custards or even small cobbler or bread pudding dishes. As a savory option, make and serve individual shepherd's pies in oval-shaped ramekins or large, round ramekins. Small ramekins make convenient vessels for serving salad dressing and dip on the side.
Sloped Sides for Custards
Custard cups come in the same standard sizes of 4, 6, and 8 ounces but have sloped or fluted sides so the top is wider than the base. These dishes more closely resemble a bowl or teacup shape. While custard cups can be made with ceramic, they are most commonly made of clear, heatproof glass. These deep cups don't have an equivalent to wide and shallow ramekins, which can be for uses other than souffles.
For Single-Serve Custards and Mixing
A custard cup allows you to bake single-serving custard dishes, including creme caramel flan. Delicate custard must be cooked in a hot water bath to regulate the temperature and prevent burning. Place the custard cups in a deep baking dish with water up to half the depth of the cups. The angled sides also make it easy to mix small amounts -- whip up 1 to 2 scrambled eggs instead of dirtying a larger bowl. Custard cups can also stand in for ramekins for all other uses except souffles and creme brulee.