Shortbread cookies are usually celebrated an all-butter affair; they have a very simple recipe, and the flavor of the butter is a central part of the cookie.

The usual recipe is something like:

Ingredients:

• 1 cup flour
• 1/2 cup butter (room temperature)
• 1/4 cup sugar
• 1 tsp vanilla extract
• 1/4 tsp salt

Steps:

• Cream the butter, sugar, and extract
• Gently mix in the flour and salt, until just combined
• Refrigerate the dough for about an hour
• Preheat the oven to 350 degrees Fahrenheit
• Roll the dough about 1/4 inch thick, and cut out cookies with a cookie cutter
• Bake for 8-10 minutes, or until just barely browned at the edges

This boils down to a very simple ratio:
4 parts flour : 2 parts fat : 1 part sugar, plus whatever additions such as vanilla extract or (my personal favorite) fresh rosemary for flavor.

A vegan version of shortbread, of course, requires substituting butter for an alternate fat, and for a cookie with these proportions, the choice is very important.

I've used two options for this: unrefined coconut oil, and Miyoko's recipe for vegan butter (which uses refined coconut oil, among other things).

Unrefined coconut oil is fatty and fragrant, and the retains all the coconut flavor, whereas refined coconut oil has a neutral, non-coconut taste.

So, let's do a side-by-side comparison of these cookies. The fats:

Left to right: vegan butter, dairy butter, and unrefined coconut oil

Cream together the fat, sugar and extract:

Add the flour and salt, and wrap in cellophane to refrigerate:

The vegan butter dough is the softest and creamiest, the butter dough a little less soft with a more robust texture, and the coconut oil dough is hard and crumbly.

Because of the texture the coconut oil dough, it's pretty much impossible to work with when its chilled, so roll and cut this dough while it's at room temperature, and refrigerate the whole prepared cookie sheet.

Unbaked:

Baked:

As you can see, the coconut oil cookies spread a little, and don't keep the crisp shape of the other two cookies.

Final verdict?

The coconut oil cookies have a light coconut flavor and are crumbly, and soft but not chewy. They're oily rather than moist, especially right after being baked, but after a day they're more manageable, and a valid choice if you don't want shortbread to be chewy.

The vegan butter cookies are almost indistinguishable from the butter cookies, although slightly less chewy by the second day.

Both substitutes work, but vegan butter will get you much closer to what the recipe expects.

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Takeaways:

Coconut oil: roll the cookies while the dough is at room temperature and refrigerate the whole cookie sheet. Don't try any fancy shapes, because they spread in the oven.

If they come out oilier than you'd like, cover them and let them sit for a day. But they'll still be oiler than is normal for a cookie.

Vegan butter: very consistent with the butter cookies, and comes out looking excellent. Best when cool but eaten sooner rather than later.

Winner: vegan butter. It's a very proper shortbread cookie.