Preheat your oven to 180°C (160°C Fan / 350°F / Gas Mark 4).
Line two baking trays with baking parchment.
In a small bowl or cup, dissolve the 2 tsp of espresso powder in 1 tbsp of boiling water. Stir well and set aside to cool slightly.
Once the coffee has cooled a little, beat the egg into the coffee mixture. Set aside.
In a large mixing bowl, add the self-raising flour and white sugar.
Add the cold, cubed butter to the flour and sugar. Using your fingertips (or a pastry blender), rub the butter into the dry ingredients until the mixture resembles fine breadcrumbs.
Pour the coffee-egg mix into the breadcrumb mixture. Mix with a spoon or your hands until just combined and a soft dough forms. Be careful not to overmix.
Cover the dough (e.g. using a reusable beeswax wrap or by placing it in a covered bowl) and chill in the refrigerator for 20 minutes. This will make it easier to roll.
Once chilled, remove the dough from the fridge. Roll the dough into 30 small, equal-sized balls (about 2.5cm or 1-inch diameter).
Place 15 balls on each prepared baking tray, leaving some space between them as they will spread a little.
Bake in the centre of a preheated oven for 8-10 minutes, or until they are pale golden and feel dry on top.
Leave the biscuits on the baking trays for a few minutes to firm up slightly, then carefully transfer them to a wire rack to cool completely.
In a small bowl, dissolve the 2 tsp of espresso powder in 1 tbsp of boiling water for the icing. Stir well and set aside to cool completely.
In a medium mixing bowl, beat the softened butter with an electric mixer (or vigorously by hand with a wooden spoon) until it's light and fluffy.
Sift the icing sugar and cocoa powder into the butter. Beat slowly at first (I am no stranger to covering myself with an inadvertent cloud of icing sugar), then increase the speed and beat until well combined and smooth.
Pour in the cooled coffee mixture and beat again until the buttercream is fluffy and even in colour.
If the buttercream is too stiff, add a few more drops of boiling water (or milk) one at a time, until you reach a smooth, pipeable/spreadable consistency. If it's too soft, add a little more sifted icing sugar.
Assemble the Mocha Kisses:
Once the biscuits are completely cool, take one biscuit and turn it so the flat bottom side is facing up.
Pipe a swirl of mocha buttercream onto the flat side, or spread a generous layer using a knife ( I tend to use a piping bag without a nozzle so you get a nice fat swirl of icing).
Gently place another biscuit on top, flat side down, to create a sandwich.
Repeat with the remaining biscuits and buttercream until you have 15 Mocha Kisses.