In the bowl combine the butter and sugar and whisk t on medium-high speed until pale and fluffy.
Add in the egg and vanilla essence, whisk until combined. Then fold in flour and pistachios till well combined.
Divide the dough in half, then roll each portion of cookie dough into a log and wrap each log tightly in plastic wrap and refrigerate for at least 30 minutes
Preheat the oven to 160-degree Celsius and prepare a baking sheet with parchment paper
Using a sharp knife, slice the logs of cookie dough into ¼" rounds.
Place the cookies ½" apart on the prepared baking sheet, and bake for 15 minutes, or until golden around the edges.
Remove the oven and place the pan on a cooling rack
Melt the chocolate on low power, in 15-second increments, stirring after each increment until completely melted and smooth.
Dip each cookie halfway into the chocolate then return to the lined baking sheet and sprinkle with finely chopped pistachios. Allow the chocolate to set before serving. To speed this up, place the baking sheet into the refrigerator.
Recipe by Maya Kitchenette at https://mayakitchenette.com/chocolate-pistachio-cookies/