Preheat the oven to 190C/170C(fan)/375F/Gas 5. Grease a 16-cup mini cake tin and line with baking parchment, or, alternatively, grease 16 chefs’ rings (5cm/2in diameter) and place them on a baking tray lined with baking parchment.
For the cakes, place the butter, sugar, eggs, flour, baking powder and coffee essence into a bowl and beat with a wooden spoon until thoroughly blended and smooth. Fold in the walnuts.
Divide or pipe the mixture equally between the mini cake tins. Bake for about 12-15 minutes, or until well-risen and the top of the cake springs back when lightly pressed with a finger.
Leave to cool in the tins for a few minutes. When cool enough to handle, turn the cakes out of the tins and remove the parchment. Leave to cool completely on a wire rack.
For the filling and decoration, beat together the butter, icing sugar, milk and coffee essence in a bowl until smooth.
When the cakes are completely cold, trim the tops if domed. Slice in half horizontally, then sandwich them together with about a quarter of the buttercream.
Spread a thin coating of buttercream around the sides of each cake and then roll the sides in chopped walnuts.
Spoon the remaining buttercream into a piping bag fitted with a small star (2D) nozzle and pipe a rose on the top of each cake. Place a chocolate-covered coffee bean over the joins.
Arrange the cupcakes on the nicest cake stand you have and serve.