Preheat the oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4. Line 2 x 12-hole muffin trays with paper cupcake cases.
Stir the cake mix, oil, water and eggs together gently in a bowl. Add the espresso coffee powder, then whisk (by hand or with an electric mixer) for 2-3 minutes, until smooth and creamy.
Divide the mixture equally between the paper cases and bake for 18-22 minutes until risen and springy to the touch. Remove from the oven and cool in the muffin trays for 10 minutes, then transfer to a wire rack and leave to cool completely; about 30 minutes.
Meanwhile, make the filling. Dissolve the espresso coffee powder in the boiling water in a small heatproof bowl. Leave to cool. Place the mascarpone cheese in a bowl and beat until smooth, then gradually beat in the cooled liquid coffee and icing sugar.
Spoon the filling into a large disposable piping bag fitted with a plain 1cm nozzle. Carefully insert the tip of the nozzle into the centre of 1 cupcake, a little way down. Gently squeeze the piping bag, pulling upward until the cupcake swells slightly and the filling comes to the top (see tip). Repeat with the remaining cupcakes.
To decorate, spoon the coffee icing into a large disposable piping bag fitted with a plain 1cm nozzle. Pipe blobs on top of each cupcake (making sure to cover the centre hole where the mascarpone filling was piped in).
Top each cupcake with a chocolate coated coffee bean and sprinkle with a little grated chocolate. Serve immediately. Store any leftover cupcakes in the fridge for up to three days.