A dumb cake, AKA a dream cake, is a cake baked to assist in certain magic rituals -- most famously, they helped a young girl dream of her future husband on the eve of one of the feast days of a virgin martyr (e.g., Saint Agnes' Eve, St. Faith’s Eve, St. Anne’s Eve) and for good measure, also on St. David’s Day, Christmas Eve, and New Year's Eve.
Dumb cakes are a subset of women's cakes, magical cakes to be eaten by women at religiously significant times. Dumb cakes, naturally, are those cakes that were made in silence. Cakes, in this case, were not necessarily sweet, and some were closer to pancakes than loaves. Some were clearly intended to be unpleasant.
"They even ventured upon the solemn and fearful preparation of the dumb-cake. This must be done fasting and in silence. The ingredients are handed down in traditional form:--"An egg-shell full of salt, an egg-shell full of malt, and an egg-shell full of barley meal." When the cake is ready, it is put upon a pan over the fire, and the future husband will appear, turn the cake, and retire; but if a word is spoken, or a fast is broken, during this awful ceremony, there is no knowing what horrible consequence would ensue!
Bracebridge Hall, Washington Irving, 1822
While the superstitions around dumb cakes are highly mutable over time, they were generally intended to be eaten before bed or in bed, perhaps with a piece left under the pillow to tempt dream husbands. The concept was sometimes extended to dumb suppers -- a silent meal -- or other periods of silence, sometimes accompanied by various foods. Often vestiges of non-Christian rituals were apparent, and the rituals could be fairly complex.
"On this day a very curious custom is observed in the North of England. A cake of flour, spring-water, salt, and sugar must be made by three maidens or three widows, and each must have an equal share in the composition. It is then baked before the fire in a Dutch-oven, and, all the while it is doing, silence must be strictly observed, and the cake must be turned nine times, or three times to each person. When it is thoroughly done it is divided into three parts. Each one taking her share, and cutting it into nine slices, must pass each slip three times through a wedding-ring previously borrowed from a woman who has been married at least seven years. Then each one must eat her nine slips as she is undressing and repeat the following rhyme:--
O good St. Faith, be kind to-night,
And bring to me my heart’s delight;
Let me my future husband view,
And be my visions chaste and true."
British Popular culture, Thomas Firminger Thiselton Dyer, 1875