Preheat oven to 190℃/170℃ fan/375℉/Gas5. Line 2 baking sheets with baking paper.
In a standmixer of electric handmixer cream butter and icing sugar well until pale and fluffy. Add the Kahlua or coffee liqueur and instant espresso coffee. Sieve in the flour and cornflour and beat well, until thoroughly mixed.
Scoop out small teaspoon sized balls and place them onto the lined trays. Make sure you have enough to make 32 biscuits, as you will need two biscuits to sandwich together.
Bake in the centre of the oven for 13-15 minutes until pale golden colour. Cool on the tray for 5 minutes and then transfer to a cake rack to cool completely.
Coffee Buttercream
In a standmixer or electric handmixer, beat until the butter is very soft and creamy.
Add the Kahlua or coffee liqueur and instant espresso coffee and beat on low speed until light and fluffy. Sandwich buttercream on one of the biscuits and sandwich together with another biscuit.