Hollow Biscuits are small, dried breads made in England.
A pale gold colour, they are similar to a rusk in texture, but in shape they look like irregular doorknobs. They have a crunchy crust with a hollow centre.
They are traditionally buttered, and served with jam or cheese. Some recommend them with potted meats as well.
A Hollow Biscuit is similar to a Dorset Knob, but Hollow Biscuits are smaller, and the dough is handled differently. For Dorset knobs, the dough is mixed, and rolled out in the palm of your hand. There is no hollow centre.
Hollow Biscuits are made from flour, salt, sugar, yeast and fat. The Independent’s food writer, Michael Batement (1932 to 2006) wrote  that they’d often be made simply from left-over bread dough enriched with a little butter and sugar. You fold the dough to make a hollow centre, then cut into pieces. These are baked, then set in a cool oven to dry and crisp.
Cromer used to be a famous brand of Hollow Biscuits. Rollings rusk shop on King Street, Great Yarmouth, used to make Norfolk Hollow Biscuits as well. One of few places still making them (as of 2010) is Merv’s Hot Bread Kitchen at 38 Market Street, Wymondham, Norfolk, England.
Store for up to 3 months.
Some speculate that the technique for Hollow Biscuits was brought over in the 700s by Danish invaders; others think it is “more recent”, a recipe evolved from contact with traders in the Netherlands and Flanders, etc., given its similarity to rusks.
Dr Zechariah Buck, choirmaster at Norwich Cathedral for 60 years from 1817 to 1877, used to lock bad-behaving choir boys in the summer house at the bottom of his garden in the Upper Close, giving them only water and Hollow Biscuits. 
In 1893, A.M. Money, Confectioner, Bread & Biscuit Baker of Suffolk ran an ad in “Beccles Almanack” calling attention to “to our Hollow Biscuits, which are unequalled in quality.”
King George VI always ordered a supply of Hollow Biscuits in when he was at Sandringham in Norfolk.
Literature & Lore
“HOLLOW BISCUITS: Mix a pound and a quarter of butter with three pounds and a half of flour adding a pint of warm water. Cut out the paste with a wine glass or a small tin and set them in a brisk oven after the white bread is drawn.” — Eaton, Mary. The cook and housekeeper’s complete and universal dictionary. Bungay, Suffolk: J. and R. Childs, 1822, Page 168.
There was a sweeter version:
“Little Hollow Biscuits: Beat six eggs, with one spoonful of rose or orange-flower water; add a full pound of loaf sugar sifted; mix these well; put flour to it that has been dried, till it is of a thickness to drop upon sheets of white paper; drop them just as they are going to be baked; sift sugar over through a lawn sieve: the oven must be slack; as soon as they are baked, take them whilst hot off the paper; dry them in the oven on a sieve; keep them in boxes, with paper between.” — Charlotte Mason. The Lady’s Assistant. London: J. Walter. 1787. Page 402.
This version was in fact stolen almost word for word from E. Smith’s 1727 “The Compleat Housewife – Or, Accomplished Gentlewoman’s Companion”
“To make Little Hollow Biskets: Beat six eggs very well with a spoonful of rosewater; then put in a pound and two ounces of loaf-sugar beaten and sifted; stir it together till ’tis well mixed in the eggs, then put in as much flour as will make it thick enough to lay out in drops upon sheets of white paper; stir it well together till you are ready to drop it on your paper; then beat a little very fine sugar and put it into a lawn sieve, and sift some on them, the oven must not be too hot, and as soon as they are baked, whilst they are hot, pull off the papers from them, and put them in a sieve, and set them in an oven to dry; keep them in boxes with papers between.”
“”As far as I know Mr Ashworth late of the Ashworth chain of bakers, is the only person to make Norfolk Knobs or, as they are locally known ‘hollow biscuits’, in Norfolk; he comes in twice a week to bake in Merv’s Hot Bread Kitchen…..the secret, according to Mr Ashworth, for Norfolk Knobs is to make a dough with cold water to stop it rising too quickly, knead it, then to turn and fold it to get the ‘hollowness’, cut it by hand into knobs. These are then baked off in a hot oven, left to cool and then finally dried out in a cool oven.” Food Lover Britain’s entry for “Mervs Hot Bread Kitchen”. Retrieved March 2006 from http://www.foodloversbritain.com/members/Mervs-Hot-Bread-Kitchen/Mervs-Hot-Bread-Kitchen/
 Bateman, Michael. Parts that taste forgot. London: The Independent. 9 January 1994.
 Kitton, Frederic G. Zechariah Buck in: A Centenary Memoir. Jarrold & Sons. 1899.
Stannard, David. Fishing Up the Moon: Norfolk Seafood Cookery: The Year of a Norfolk Longshoreman. Larks Press, 2005. Page 101.