Preheat the oven to 180℃/160℃ fan/350℉/gas mark 6. Line two baking sheets with baking paper.
Mix the flour and sugar together in a bowl. Add the cold butter and rub it in using your fingertips until it resembles fine breadcrumbs no lumps of butter remain.
Place the instant espresso powder or coffee granules in a small bowl and mix in a teaspoon of boiling water to dissolve it then stir in the milk.
Scrape the coffee mixture into the flour mixture and stir with a spatula to combine. Use your hands to briefly knead the dough until it is smooth and no longer looks patchy.
Roll the dough into ½ Tablespoon balls and place them spaced apart on the lined baking sheets, making sure that they have room to spread. You should get around 34 balls.
Bake for 12-20 minutes depending on whether you want them soft or crunchy ( I think 15 minutes is perfect). Allow to cool on the baking sheets for a couple of minutes then transfer to a wire rack to cool completely.
To make the buttercream, whisk together the softened butter and icing sugar with an electric mixer. Mix the coffee powder with ½ teaspoon of boiling water and add it to the bowl. Continue to whisk for a good few minutes until very light and fluffy.
Spread or pipe a little buttercream onto the underside of half of the cooled biscuits. Top each one with another biscuit and serve.