The ratio, and what is not in it
Shortbread is sugar, butter and flour, with a pinch of salt. That is the whole list. The proportions are 1 : 2 : 3 by weight, meaning one part sugar, two parts butter and three parts flour, and some sources give three to four parts flour. The ratio is quoted both ways round; 3:2:1 flour-first is the same thing.
What matters as much as the ratio is the absences. There is no egg and no leavening. Nothing lifts it, and nothing binds it but the butter itself.
Set that against the doughs specified elsewhere on this site and shortbread takes a distinct position. Pâte sucrée and pâte sablée are both bound with egg. Pâte brisée is bound with cold water. Shortbread has no liquid binder at all, which is precisely why it is sandy rather than crisp or flaky, and why it crumbles instead of shattering.
The butter is the flavour, and that is not a figure of speech
With no egg, no leavening and no liquid, there is nothing in a shortbread to hide behind. Everything it tastes of comes out of the butter. It is the one biscuit on this site where the quality of the butter is not a refinement but the substance of the result.
It also explains why shortbread is so unforgiving of substitution. In a dough bound with egg, changing the fat changes one contributor among several. Here it changes the only one. A shortbread made with a low-fat spread is not a slightly different shortbread; it is a different and much worse biscuit, because the ingredient carrying the flavour has been replaced.
What cinnamon does in a dough this bare
Adding a spice to shortbread is a different proposition from adding it to a richer dough. There is no egg to carry it, no liquid to disperse it and no strong competing flavour, so a cinnamon shortbread tastes markedly more of cinnamon than the same quantity would in a cake batter. The bareness of the dough amplifies whatever goes into it.
The corollary is that it amplifies mistakes the same way. Too much cinnamon in a shortbread reads as harsh rather than warm, and there is nothing in the formula to soften it. What quantity is right is not established here, because nothing consulted for this page specifies a dose, so no figure is given rather than an invented one.
The three pages that rest on this dough
Shortbread turns up on this site three times, always as a base carrying something else, and it is the same base each time. This page is the spice worked into the dough. At apple pie shortbread it carries a wet fruit layer, which raises a problem the tart doughs have a measured answer to. At lemon squares it carries a set topping, and what sets that topping is not what sets a jam.
The dough it is most often compared with, pâte sablée, is specified at pâte sucrée and pâte sablée, where the comparison runs the other way, since that page describes sablée as shortbread-like. The full set of doughs, creams and specifications is in the technique section.
Questions about shortbread
What is the shortbread ratio?
One part sugar, two parts butter, three parts flour by weight, with a pinch of salt. Some sources give three to four parts flour, and the ratio is often quoted flour-first as 3:2:1, which is the same thing.
Does shortbread contain egg?
No, and that is what distinguishes it. Pâte sucrée and pâte sablée are bound with egg and pâte brisée with cold water; shortbread has no liquid binder at all, which is why it is sandy rather than crisp or flaky.
Is there baking powder in shortbread?
No. There is no leavening of any kind, so nothing lifts it. The texture comes entirely from the fat-to-flour proportion.
Why does the butter matter so much in shortbread?
Because there is nothing else. With no egg, no leavening and no liquid, everything the biscuit tastes of comes out of the butter, which is also why substituting the fat changes the result far more than it would in a richer dough.
How much cinnamon should go into a cinnamon shortbread?
Not established here, because nothing consulted specifies a dose, so no figure is given. What is worth knowing is that a bare dough amplifies a spice: the same quantity reads much stronger here than in a cake batter, in both directions.