What Sugar to Use in Shortbread

Cookbooks from the British Isles usually call for "caster sugar" in shortbread.  From what I've read online, the sugar sold as "superfine" here in the States is comparable.  I was happy to locate this superfine sugar at one of our local grocery stores.

This week's experiment was to bake two batches of shortbread, one using 1/3 cup powdered sugar, the other using 1/3 cup superfine sugar.  The other ingredients in both batches were the same:  1/2 cup butter, 3/4 cup all purpose flour, and 1/4 cup white rice flour.

There was no mistaking the two batches!  Check out the photos below.  The batch with superfine sugar "fell", and had a very different, almost chewy texture. It did not hold its shape when cut and was difficult to remove from the pan.   It tasted sweetest, but that didn't make up for the texture.  Do you like how I label the batches- one chocolate chip for Batch 1, two chocolate chips for Batch 2?

I have to get all the shortbread out of the house!   This week it goes to work with Skip.  I embellished the wedges with a lemon/powdered sugar glaze, and tiny bit of lemon zest.  They're a tasty blend of sweet and tart.