Bob's Red Mill has come to my rescue! They make a biscuit and baking mix that is free of both gluten and potato starch.
4 cups gluten-free flour
1-1/4 cups confectioner's or caster sugar
1/4 teaspoon of salt
1 pound of unsalted butter
Preheat oven to 325°F (162°C).
Cream the sugar and butter together until light and creamy. Add the flour and salt and knead until combined.
Place in baking pan. The dough should not be more than 1/2 inch thick. I use two round (8-inch and 10-inch) springform pans. Bake for 20-25 minutes.
Decorate if desired. I have been known to use decorator gels and/or colored sugar.
This was a little gritty and dry immediately after baking. The soy flour was definitely there in flavor, but was not overpowering as I had feared.
I took the shortbread to a gaming night a couple of nights later. The moisture levels in the shortbread had equalized nicely. While it was a little more crumbly than I prefer, it was no longer as dry and gritty as it was right after it was baked. And the shortbread tasted better than when it was first out of the oven. I think I want to increase the fat and sugar a little. I want the shortbread a little moister and sweeter, but for those who don't like overly sweet shortbread, this should be just about perfect.