An hour or two before you start baking, get out your butter. It needs to be very soft before you start baking.

Lightly butter an 8-by-8-inch light-colored metal baking pan (dark or non-stick finishes will brown the cookies too quickly). Line pan with parchment paper, leaving overhangs on all 4 sides (see here for parchment-lining techniques). Lightly butter the parchment.

Place softened butter in a mixing bowl. Add salt and blend with a wooden spoon until completely incorporated (you can use an electric mixer here if you like, but I find it unnecessary assuming your butter is sufficiently softened). It should be the consistency of thick mayonnaise.

Add sugar and mix well. (Note, this recipe calls for superfine sugar, which you can achieve by pulsing granulated sugar in a blender or food processor for a few seconds. The finer texture helps produce a more tender texture.)

In a bowl, whisk together flour, pistachio flour, cornstarch, and matcha until evenly incorporated. Add to butter along with pistachio extract and mix until dry ingredients are just incorporated, scraping down your bowl and scrape off your spoon often to make sure all the bits of butter are accounted for.

Dump dough into prepared baking pan, and press into an even layer. You can lightly flour your hands if necessary, or place a piece of plastic wrap or parchment down on top of the dough to help you smooth it into an even layer without it sticking to your hands.

Press a layer of plastic wrap down on top of the dough and refrigerate for 20 to 30 minutes.

While dough is chilling, preheat oven to 300 degrees F.

Bake for 45 to 50 minutes or until top is matte and edges are just barely (and I mean barely) starting to brown.

Remove from oven and place pan on a wire baking rack. Let cool for 15 minutes, then sprinkle with remaining granulated sugar. Tilt the pan to make sure the sugar evenly covers the entire surface, then dump out the excess.

While the cookies are still slightly warm, use a thin knife or a bench scraper to cut your bars, wiping off the knife with a towel between slices. This shortbread cuts more cleanly while it is warm, but then you have to let it completely cool before removing the bars from the pan (use the parchment overhangs to gently lift out the entire slab).

Cookies can be stored in an airtight container at room temperature for up to 5 days.