1. In a large bowl, beat the softened butter and powdered sugar until light and creamy, about 2 minutes. Beat in the vanilla.

2. Whisk the flour and salt together, then stir them into the butter mixture until a soft dough forms.

3. Fold in the chopped pistachios and chopped dark chocolate until evenly distributed.

4. Divide the dough in half and shape each half into a log about 2 inches wide. Wrap in plastic wrap and chill for at least 2 hours, until firm.

5. Preheat the oven to 350°F. Line two baking sheets with parchment paper.

6. Slice the logs into 1/2-inch rounds and place them about 1 inch apart on the baking sheets.

7. Bake for 10 to 12 minutes, until the edges are lightly golden. Cool on the pans for 5 minutes, then move to a wire rack to cool completely.

8. Melt the dipping chocolate in a heatproof bowl over simmering water or in the microwave in 30-second bursts, stirring until smooth.

9. Dip half of each cooled cookie into the melted chocolate and place on parchment paper. Sprinkle immediately with chopped pistachios and a pinch of flaky salt, if using.

10. Let the chocolate set at room temperature or in the refrigerator for about 30 minutes.