Grease a 28 x 18 cm (11 x 7 inch) baking tin.
Beat together the butter and sugar in a large bowl until light and fluffy. Stir in the flours, chocolate drops or chopped chocolate and pistachios until well combined.
Press the mixture into the prepared tin and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes until golden.
Remove from the oven and mark into 12 triangles, then transfer to a wire rack to cool completely before removing from the tin. (It is sometimes easier to remove the shortbread from the tin once it has been chilled a little.) Drizzle with the melted chocolate. Leave to set, then separate into triangles.