Combine the softened butter, caster sugar, lemon zest, and salt in a mixing bowl. Mix until well combined. Add the beaten egg, followed by the flour and baking powder. Knead the dough until smooth, then wrap it in plastic wrap and refrigerate for at least 1 hour.
Preheat the oven to 175°C. Line a baking tray with baking paper. Briefly knead the chilled dough to soften it, then roll it out on a lightly floured surface to a thickness of 5 mm. Use a 6 cm round cutter to cut out circles from the dough. Arrange the circles on the prepared baking tray.
Brush the dough circles with the beaten egg and generously sprinkle each with roasted peanuts. Lightly press the peanuts into the dough, then brush the cookies again with egg to ensure the peanuts stick.
Bake the cookies in the preheated oven for 15–20 minutes, or until golden brown. Transfer them to a wire rack to cool completely.
To temper the chocolate, heat the dark chocolate drops in the microwave in 20-second intervals, stirring after each heating, until fully melted and smooth. Be careful not to overheat.
Dip half of each cooled cookie into the tempered chocolate. Place the dipped cookies on baking paper and let the chocolate solidify at room temperature.