Microwave 1/2 cup semi-sweet chocolate chips and 1 tablespoon unsalted butter in a heatproof bowl in 30-second intervals until melted, 1 to 2 minutes; stir until smooth. Spread the melted chocolate over the lebkuchen, smoothing the top with a small offset spatula or the back of a spoon. Chill until firm, about 20 minutes. Microwave 2 tablespoons of milk chocolate chips, if using, in a heatproof bowl in 30-second intervals until melted, 30 seconds to 1 minute. Use a spoon, fork, or a small piping bag to drizzle the milk chocolate over the lebkuchen. Chill until firm, about 10 minutes, before cutting into small bars or squares.