1. Preheat oven to 195C (175C fan-forced), grease and line the base and sides of a 20cm round cake tin with baking paper.

2. In a small bowl, combine the coffee in the water and stir until dissolved. Set aside to cool. Place the butter and sugar in the bowl of a cake mixer fitted with the paddle attachment and mix on medium-high speed until light and fluffy. Sift flour and salt together, add to the butter mix, alternating with the egg. Add whiskey (or vanilla) and coffee, combine on low speed.

3. Scrape the mixture into the prepared cake tin and bake in the centre of the preheated oven for about 45 minutes, or until a skewer inserted into the middle comes out clean. Place the cake on a rack to cool completely.

4. To make the Bailey's filling, place all the ingredients in the bowl of a cake mixer fitted with the whisk attachment and beat on low speed until combined. Increase the speed to medium-high and whisk until the cream is thick enough to spread. Refrigerated until needed.

5. To make the coffee icing, combine coffee and warm milk in a medium-sized bowl. Add the icing sugar to the coffee mixture, together with the soft butter. Whisk until smooth and thick, then add the whiskey if using.

6. To serve, slice the cake in half horizontally and fill with the Bailey's filling. Transfer to a serving plate and spoon the coffee icing all over the top of the cake, allowing it to drip unevenly down the sides. Dust lightly with cocoa powder before serving. The cake is best served on the day it's made.