Meanwhile, for the buttercream, whisk the butter with a handheld electric whisk until light and fluffy. Gradually add the icing sugar on a low speed, then mix on high speed for 5 minutes. Whisk in the Irish cream liqueur the set aside until needed.
When the cake is completely cool, cut in half to make two rectangles about 17.5×12.5cm/7x5in each. Spread a thin layer of buttercream on top of one rectangle. Place the other on top. Freeze for 10 minutes to firm up the icing.
Remove the cake from the freezer. Trim the edges square with a sharp serrated knife, then measure a grid to divide the cake into 3.5cm/1¼in squares. Carefully cut the sponge into individual fancies. Return the cakes to the freezer for at least 20 minutes until they are firmly frozen.
Working with three fancies at a time, spread the buttercream thinly, but smoothly, around the sides and top before returning them to the freezer. Repeat with the remaining fancies, ensuring that they all are in the freezer for 20 minutes before applying the fondant.
For the chocolate fondant, melt the dark chocolate in a small bowl in the microwave or over a pan of simmering water. Allow to cool slightly.
Mix the sifted icing sugar in a large heatproof bowl with 125ml/4fl oz water and the glucose syrup. Place the bowl over a pan of simmering water. Stirring constantly, cook the fondant until it reaches 45C. Remove from the heat and allow it to cool to 38C. Stir in the melted chocolate. Place the bowl back over the heat to maintain a temperature of 38C – this is warm enough to pour smoothly but cool enough not to slide off the fancies.
Place a large thin-wire cooling rack over a baking tray to catch any drips of fondant. Take each fondant from the freezer and insert a fork horizontally in the bottom layer of sponge. Hold the fancy over the bowl of fondant and generously spoon over the fondant to completely cover the sides and top. Gently tap to shake off any excess icing, then place on the cooling rack to set, using another fork to push the fancy free.
When set, carefully lift the fancies from the cooling rack with a large offset palette knife. Decorate the top of the fancies with a swirl of buttercream, piped lines of white chocolate or chocolate coffee beans, as desired.
