Line a baking tray with baking parchment. Tip the hazelnuts into the bowl of a food processor and pulse until finely chopped. Add the flours, sugar and a pinch of flaked sea salt, and process for 20-30 secs until fully combined. Add the butter and pulse until the mixture just starts to come together. Tip the dough out onto a work surface and knead by hand until smooth.
Roll the dough into 25 small balls, then transfer to the prepared baking tray. Using your thumb or the handle of a wooden spoon, make an indent in the centre of each piece of dough. Put the tray in the fridge and chill for 30 mins before baking. Heat oven to 180C/160C fan/gas 4.
Bake in the oven for 15-20 mins or until light golden brown. Put the chocolate and oil in a heatproof bowl and set over a pan of simmering water, stirring occasionally, until fully melted. Use a teaspoon to top each cookie with a little melted chocolate. Put aside until the chocolate has set.