Mix the espresso powder with 1 tsp hot water to form a paste (you’ll think it isn’t enough water but it is – don’t add more otherwise it’ll be too watery). Set aside.
Beat the butter and sugar until soft and blended, then beat in the coffee mixture. Don’t overbeat the mixture as you don’t want to incorporate too much air. (If the coffee isn’t completely incorporated, don’t worry – it will be once you add the flours).
Sift together the flours and add them to the butter in three lots, mixing in with a wooden spoon and, eventually, your hands. Bring together into a ball, then transfer to a lightly floured surface and quickly knead in the hazelnuts. Don’t overdo this – you don’t want the butter to get too warm. Press the dough into a circle, wrap it in cling film and put in the fridge for about 40 mins.
Roll out the dough on a lightly floured surface to 0.75cm thick. Cut out circles about 6cm across with a biscuit cutter and put the circles on a non-stick baking sheet (or two baking sheets, depending on their size). Re-roll the dough as you need to, but try not to do this too often. Put the baking sheets in the coldest part of the fridge and leave them for about 45 mins. Heat oven to 160C/140C fan/gas 3.
Bake the shortbread for 20 mins, or until the top is just firm when you press it with your index finger. Take out of the oven and leave on the baking sheets to cool for a while (it is very fragile when hot), then transfer to a wire rack to cool completely.
Melt the chocolate in a bowl set over a pan of simmering water, then remove the bowl and leave to cool a little and rm up somewhat. Dip each piece of shortbread halfway in the melted chocolate, then put on a sheet of baking parchment. Sprinkle the toasted hazelnuts on the chocolate half and leave to set completely. Will keep for 4 days in an airtight tin.