Preheat the oven to 180°C (gas mark 4, 350°F) and line a baking tray with parchment paper.
Add butter and caster sugar into a large mixing bowl and cream together until light and fluffy.
In a cup, mix a tablespoon of coffee with a tablespoon of warm water until the coffee granules have dissolved.
Then add an egg into the mixing bowl with the dissolved coffee and mix together until evenly distributed.
Pour the self-raising flour into the bowl and mix together until the biscuit dough has formed.
Scoop a teaspoon of dough, place it in the palm of your hands and roll it into a ball, then place the dough on a lined baking tray. Repeat this step for the rest of the dough.Leave space between each ball as they will spread whilst baking in the oven.
Bake for 15 minutes in the oven, then leave the biscuits to cool down.
To make the buttercream, mix together a teaspoon of instant coffee with a teaspoon of water and set aside for later use.
In a mixing bowl, add butter and icing sugar and mix together until combined. Then add the dissolved coffee and mix together until evenly distributed.
To assemble the biscuits, spread or pipe the coffee buttercream on top of one biscuit, then top with another biscuit.