• Preheat the oven to 320°F/160ºC. Line two large baking sheet with non-stick parchment paper.
• Beat the unsalted butter and granulated white sugar together until smooth and creamy.
• Add the ground hazelnuts, flour and salt to the creamed butter and sugar. Combine until the dough comes together. Be sure that the mixture is well combined by scraping the bowl with a spatula, getting right to the bottom. Mix again if necessary.
• Divide the dough into four equal portions. Roll each portion into a 9-inch/22 cm log. Cut the log into 25 even slices. Roll each into balls. Each ball will be approximately the size of a hazelnut in the shell. If the dough is too sticky, chill for 30 minutes.
• Arrange on prepared baking trays, leaving a little room between each. Chill for 1 hour.
• Bake in the preheated oven for 15 minutes, until pale golden.
• Remove from the oven. Cool for 5 minutes before transferring to wire rack to cool.

To assemble

• Place the chopped chocolate in a microwave safe bowl.
• Melt chocolate in the microwave using short 30-second bursts on low power, stirring well after each interval.
• Stop before fully melted and let residual heat finish the job. Cool until slightly thickened before using.
• Allow the cookies to cool completely, then spoon half a teaspoon of melted chocolate on one upturned cookie. Allow the chocolate to set a little. Top with a matching half and sandwich them together.
• Allow chocolate set before serving. If necessary, place in the fridge to set.