I get excellent results with brown rice flour, and recommend it for its better nutrition, but white rice flour is fine, too.
On the one hand, there is no way to improve on traditional Scottish shortbread (with salt, please: just like real bread, shortbread without any salt is longer in shortcomings than in shortening). On the other hand, this recipe gives some of the most “more-please” biscuits I’ve ever tasted. To the plain delights of butter, sugar and flour (all-purpose and a little rice flour, for extra crunch), they add the subtle nuttiness of ground oats and the fruitiness of jam made ever so slightly chewy in the center by baking — and don’t forget the lovely addition of mace, which has both citrusy and nutmeg-y taste notes. (If you don’t have mace on hand, feel free to substitute nutmeg.) Crumbly, mildly crunchy, melty, and chewy: this how we have it “all ways” with this shortbread. You can use any kind of jam — from peach to blackcurrant — but if the jam has thick fruit pieces, you’ll want to pick them out as you delve into the jar.
Whole mace blades, which I like even better than the nutmeg they once contained.
5 oz butter, softened
4.5 oz unbleached all-purpose flour
1.5 oz rolled oats, to grind
1.5 oz brown or white rice flour [if you don’t want to buy a bag, just grind your regular uncooked rice in a spice blender or other blender, to a fine powder]
2 oz superfine granulated sugar
½ teaspoon ground mace (or 1 mace blades/aril, ground, if using)
1 teaspoon salt
Jam for spreading over half the dough
To bake:
In a large mixing bowl, cream the butter with a spoon. If the butter is stiff, soften in the microwave in a suitable bowl for up to 30 seconds, on a low setting such as no. 4.
Grind the oats in a mortar, spice grinder, or food processor so that you’ve got gritty bits rather than slices. Stir into the butter the ground rice and oats, mace, sugar, and salt, and combine well.
Then add the all-purpose flour (adding it later just helps keep the dry ingredients from jumping out of the bowl).
Chill the dough for 5-10 minutes.
On a parchment-lined baking tray, press or roll dough out ¼” thick, into a rectangle. Chill again briefly if dough seems very soft or butter seems to be melting.
Using a knife or bench scraper, cut the rectangle of dough in half.
Spread one half with jam, coating it.
Place the other half directly on top of the jammy half. Now cut this sandwich into strips or fingers about an inch wide. You should have about 10 strips. Dip the blade in water between cuts if you find the knife is not drawing out cleanly. (There is no need to prick the tops, though most shortbread recipes ask you to do this.) Then draw the blade through the middle to cut each strip in half. Now you have 20 fingers.
Put the tray in the fridge to chill and firm up for 10-15 minutes.
Preheat the oven to 300°F. Bake for about an hour on the middle rack.
Allow to cool before serving, or store shortbreads in an airtight container for a few days, or in the freezer.