These walnut-topped finger biscuits have two fillings: a bitter-chocolate ganache, and a coffee-amaretto buttercream.
Heat the oven to 190°C/170°C fan/375°F/Gas 5. Draw 30 7cm-long lines on the underside of the paper lining your baking sheets, to use as a piping guide.
Put the butter in a mixing bowl and beat until creamy. Sift in the icing sugar, cornflour, flour and coffee. Add the vanilla, then combine to a soft dough.
Transfer half the mixture to the piping bag with the large star nozzle, and the other half to the bag with the round nozzle. Pipe 15 biscuits each 7cm long with each bag, piping along the lines you’ve drawn on the baking paper.
Bake the biscuits for 13–15 minutes, until golden. Rotate after 10 minutes for an even bake. (You may need to bake in 2 batches.) Remove from the oven and cool on the baking sheets on a wire rack, until firm.
Meanwhile, make the ganache. Put the chocolate in a heatproof bowl. Heat the cream in a pan until almost boiling, then pour it over the chocolate. Leave for 1 minute, then stir to melt the chocolate. Stir in the butter. Cover and chill for 10 minutes, until cool but still fluid.
Now make the buttercream. Beat the butter in a mixing bowl until light and fluffy. Stir in half the icing sugar, beat to combine, then add the remaining icing sugar and three quarters of the coffee and all the liqueur. Beat until light and fluffy. Taste, and add more coffee or liqueur as needed. Transfer to a small piping bag.
Remove the ganache from the fridge, beat with a wooden spoon for 3–4 minutes, until fluffy and thick, and transfer to a second small piping bag.
To assemble, remove the biscuits from the baking sheets. Arrange the plain biscuits, underside up, on a board. Snip the end of the buttercream piping bag. Pipe a 1.5cm dot of buttercream on the centre of each. Repeat with ganache on either side of the buttercream (you could use a star nozzle for variation, if you prefer). Set a ridged biscuit, underside down, on top of each to sandwich.
To decorate, put the melted chocolate in the third small piping bag. Snip the end and pipe a streak of chocolate along the top of each finger, scatter over the nuts and leave to set. Transfer to a serving platter.