Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large bowl, beat butter and sugar until light and fluffy.
Add egg and vanilla; mix until smooth.
Stir in flour, oats, baking soda, and salt until just combined.
Fold in chopped pistachios.
Scoop 2 tablespoons of dough per cookie onto the baking sheet.
Bake for 10–12 minutes, until edges are lightly golden. Let cool completely.
Melt chocolate in the microwave in short bursts, stirring until smooth.
Dip cooled cookies halfway into chocolate; place on parchment paper.
Sprinkle with extra pistachios before the chocolate sets. Let firm up before serving.