Preheat the oven to 350°F. Spray a 9-inch square cake pan with cooking spray and line with a parchment paper sling.

In the bowl of a food processor, add the pistachios and pulse until pistachios are finely ground. Transfer to a medium bowl.

Add the flour, cocoa powder and salt to the pistacho mixture. Stir well until combined.

In the bowl of your stand mixer fitted with the paddle attachment, combine the butter, sugar and vanilla. Beat on medium speed for 2 minutes or until light and fluffy.

Add the flour mixture to the butter mixture and mix just until combined.

Transfer the mixture to the prepared baking pan. Using a small pastry rolling pin or your hands, press the dough into the bottom, making sure to get to the edges and corners.

Place in the refrigerator for 10-15 minutes or until the dough is slightly firm.

Using a knife, cut the dough into 24 strips. Using a toothpick, prick each strip 3-4 times.

Bake in the preheated oven for 35-37 minutes, or until set and slighly firm to the touch. Remove from the oven and immediately trace back over the cut marks with the knife. Allow the bars to cool completely in the pan.

Lift the bars out of the pan using the parchment paper sling and separate them into bars and place them close together on a wire rack with a sheet of pachment paper underneath.

Melt the chocolate according to package instructions. Using a spoon, carefully drizzle the tops with the melted chocolate, sprinkle with chopped pistachios and flake salt. Allow the chocolate to set before eating.