Using an electric mixer on medium speed, beat the butter and sugar until light and fluffy. Scrape the sides of the bowl with a spatula as needed. Add the egg and vanilla, and continue mixing.

Add the flour and pistachios, and mix until fully incorporated. The dough will look crumbly but will hold together when pressed. Do not over-mix.

Place the dough on a lightly floured surface and gather into a ball. Divide the dough in half, and roll into two equal-sized logs approximately 2-3 inches in diameter. Tightly wrap each log individually in plastic wrap. Chill for 2 hours.

Slice the chilled logs into 1/4-inch thick rounds, and place them on the prepared pans.

Bake one pan at a time for 15 to 18 minutes, or until the edges begin to turn a golden brown. They will firm as they cool, so don't over-bake. (Keep the remaining dough chilled between batches.)

Place the pans on cooling racks, and allow the cookies to cool completely.

Once the cookies are cool, melt the chocolate in the microwave at half-power in 30-second increments, stirring between each interval until melted.

Drizzle each cookie with melted chocolate and sprinkle with salt or chopped pistachios. Allow the chocolate to set before serving.