Soak the shelled pistachios in water for 30 minutes, then drain and air-dry them on a paper towel.

In a mixer, beat the softened butter with granulated sugar until fluffy. Add the egg and mix well. Gradually add flour and dried pistachios until a stiff dough forms.

Lightly flour your work surface and knead briefly. Divide the dough in half and roll into logs about 2 inches in diameter. Wrap in plastic wrap and refrigerate for at least 2 hours.

Preheat oven to 350°F. Slice logs into 1/4-inch rounds and place on parchment-lined baking sheet. Bake for 10-12 minutes until edges are lightly golden. Cool for 15 minutes.

Melt dark chocolate in a double boiler or microwave. Dip each cookie halfway into chocolate, sprinkle with sea salt or chopped pistachios if desired, and allow to set.