American cookies with pistachios

230 g all-purpose flour 40 g cocoa powder ½ tsp baking soda ½ tsp salt 200 g unsalted butter, at room temperature 100 g brown sugar 100 g white sugar 1 large egg 1 tbsp vanilla extract 100 g white chocolate, chopped 100 g milk chocolate, chopped 100 g dark chocolate (couverture), chopped 50 g pistachios, roughly chopped Flaky sea salt (optional)