The nuns at Convento de San Leandro in Seville, Spain, have sold just one item for more than four centuries: yemas de San Leandro. These rich, creamy nuggets are a simple mixture of sugar, lemon juice, and egg yolks.

Establishments across Spain make traditional yemas (diners enjoy yemas de Santa Teresa, for example, on October 15, the saint’s feast day). But to acquire San Leandro’s signature supply of yemas, you’ll have to visit the convent, a Seville institution since the 13th century.

When you enter the foyer, you’ll notice a revolving tray embedded in a wooden door. Reference the price list, then place the appropriate amount of money on the tray and rotate it behind the wall. A few moments later, a box of wrapped yemas should appear in its place. You’ll have to put your faith in San Leandro’s residents, but this shouldn’t prove too difficult. You’re dealing with nuns, after all.