2/3 cup rice flour or substitute cornstarch if rice flour is unavailable
½ teaspoon salt
1 ½ cups (¾lb) fresh unsalted butter, at room temperature
6 tablespoons fruit sugar or superfine sugar
6 tablespoons tightly packed light brown sugar
¾ cup miniature butterscotch chips
¾ cup English toffee pieces (available in the baking sections of most supermarkets)
Additional unsalted butter for greasing the pan
1. Preheat oven to 325°F. Butter the bottom and sides of a 9x13-inch metal baking pan. Line the bottom and up the two long sides with a piece of parchment paper. Leave about a 1-inch overhang over the sides to make removing the cooled shortbread easier. Sift the all-purpose flour and rice flours together with the salt and set aside.
2. In the bowl of an electric or stand mixer fitted with the paddle attachment, or in a large mixing bowl with a wooden spoon, beat the butter until very smooth. Gradually add the sugars and cream the mixture until it is very light and fluffy. If using a mixer, transfer the creamed butter-sugar mixture to a large mixing bowl. Add the flour mixture, about ½ cup at a time, fully incorporating each addition before adding the next. Use your fingers to knead the final portion of dry ingredients into the dough, keeping your palms off the dough as much as possible, so the warmth doesn’t turn the butter oily. When the last of the flour is fully blended, add the butterscotch and toffee bits and knead them into the dough until they are evenly distributed.
3. Press the dough firmly into the prepared pan and use the back of a metal spoon to smooth the surface. Prick the dough all over with a fork and set the pan in the center of the oven. Bake the shortbread for about 45 minutes, then prick the dough again to release any trapped air. Return the pan to the oven for another 15 or 30 minutes, or until the edges are light golden brown, and the center feels just firm to the touch.
4. The shortbread will set to a very firm biscuit as it cools, so it must be cut while it is still warm. Cool the pan on a wire rack for 7 or 8 minutes, then run a sharp paring knife around the outside of the dough to loosen the edges. Make two long cuts in the shortbread, dividing it evenly into three rectangles, each cut beginning and ending at a short side of the pan. Cutting from long side to long side, cut the rectangles into about ¾-inch wide fingers, wiping the knife on a clean towel between each cut, as it gets sticky and can pull and tear the cooling shortbread.
5. Leave the fingers to cool completely in the pan, then re-cut and transfer them to airtight tins. This shortbread can be frozen before or after it is baked. Freeze the dough pressed into the prepared pan, well wrapped with plastic and aluminum foil. Thaw overnight in the refrigerator, without disturbing the wrapping, and bake directly from the refrigerator. The baking time may have to be increased by a few minutes to compensate for the chilled dough. Freeze the cooled fingers in airtight bags or containers, layering between sheets of waxed or parchment paper and wrapping the whole tin or container with aluminum foil. Thaw the entire package, without removing the wrapping, at room temperature for 6 to 8 hours.