First, roughly chop the couverture and pistachio nuts. Now stir the butter, sugar and salt until frothy. With the mixer, on the highest level for about 5 minutes. Now add the egg and stir in until the mixture is bound. Add the flour and the lime zest one after the other. Gradually stir the rum, couverture and pistachios into the batter.
Cover the baking sheet with parchment paper. Preheat the oven to 180 degrees top / bottom heat (convection 160 degrees).
Use a teaspoon to place piles the size of a walnut on the baking sheet. Leave a larger space (approx. 4-5 cm) between the cookies as they will diverge.
Bake the cookies golden for 8-10 minutes and leave to cool on a wire rack.
Layer the cooled cookies in tin cans. Place between the layers of parchment paper. In this way they can be kept cool and dry for approx. 3-4 weeks.