Preheat the oven to 180°C, fan 160°C, gas 4.
Beat the butter and sugar together in a stand mixer or with an electric hand whisk for a few minutes until well combined. Add the beaten egg and mix again.
Tip in the flour, baking powder, half the popcorn and a pinch of salt; mix together.
Roll half the dough into golf ball-sized balls. Put them on a large baking tray lined with baking paper and lightly flatten each one. Press half the remaining popcorn into the cookies. Bake the cookies for 12-13 minutes or until lightly golden. Leave on the tray for 2-3 minutes, then transfer to a wire rack to cool. Repeat with the other half of the dough and the rest of the popcorn.
Melt the chocolate in a bowl set over a pan of barely simmering water, then drizzle over the cookies. Leave to cool and set (you can pop them in the fridge to speed up setting the chocolate).
