Preheat the oven to 350 degrees F.

Butter a 9 1/2 inch pan with removable bottom, then sprinkle it with flour. Set aside.

Combine butter, sugar, cocoa, and salt in a medium size bowl. Beat with an electric mixer, starting on low at first then gradually increase the speed to medium, mixing until just combined. Scrape the sides of the bowl. Mix briefly.
Add vanilla extract and egg yolk.

Add the flour and mix the ingredients on low speed.Mix until the dough clumps, takes about 1 minute or so. Scrape the sides of the bowl. Mix briefly.

Transfer mixture into the pan, press gently into the bottom. No need to go up the sides, just cover the bottom evenly.

Place in the oven on the middle rack, and cook for about 20 to 25 minutes. You want the top of the shortbread to lose it's wet appearance, and the dough to slightly pull away from the sides of the pan a little bit. That lets you know it's done. It took 20 to 21 minutes for us

Remove from oven. Set aside.

Use a double boiler for the glaze. You want to fill a small saucepan with water about 2 to 3 inches up the sides. Place over medium heat, allow to warm up, lower to a medium-low heat and place a heat proof glass bowl over top of the pan, ensuring the bottom of the glass bowl does not touch the top of the water.

Place the butter and chocolate in the bowl and allow to melt. Whisk occasionally, until the mixture is smooth.

Pour the warm glaze over top of the shortbread, then spread evenly with an offset spatula.

Toss the pistachios over top.

Allow the glaze to set up by setting aside until it cools.

Now remove the shortbread from the pan and cut into wedges. You can make them large or small, your choice.

Serve this at room temperature.

Serve with a big old chocolate loving smile!