Preheat the oven to 190C/fan 170C/gas 5. Line 2 baking sheets with baking parchment and draw 8 x 10cm lines, 2.5cm apart on each sheet. Turn the parchment over so the ink/pencil line is underneath but can be seen, to use as a template.
Measure the butter and icing sugar into a bowl and beat well until pale and fluffy. Add the coffee liqueur and espresso powder. Sieve in the flour and cornflour and beat well, until thoroughly mixed.
Spoon the mixture into a piping bag fitted with a medium star nozzle. Pipe the mixture in 16 straight lines along the lines on the baking parchment.
Bake in the centre of the oven for 13-15 minutes, until a pale golden-brown colour. Cool on the baking tray for 5 minutes then transfer to a cooling rack to cool and harden.
For the buttercream, cut the butter into cubes and place in the bowl of a stand mixer. Beat on medium speed until the butter is very soft and creamy, scraping down the sides of the bowl occasionally.
Add the icing sugar, coffee liqueur and instant espresso powder and beat on low speed to avoid an explosion of icing sugar, then beat on medium speed until light and fluffy. Spoon ¾ of the mixture into a piping bag fitted with a ribbon nozzle and the remaining ¼ into a piping bag fitted with a star nozzle.
To decorate, melt the chocolate in a bowl set over a pan of simmering water. Pour the melted chocolate into a ramekin, then dip one end of 8 of the biscuits into the chocolate. Place on a wire rack, then chill in the fridge until the chocolate has set.
Pipe ribbons of buttercream along the flat edge of the remaining eight biscuits and sandwich together with the chocolate dipped biscuits.
Pipe a rosette of buttercream (using the star nozzle) on top of the dipped chocolate and top with a chocolate-coated coffee bean.