For the sponge: Preheat your oven to 180C/160C Fan/Gas Mark 4 and grease and line a 12x9inch baking tin. In a small bowl, stir together the cocoa powder and freshly brewed black coffee to a thick paste without lumps. Then set aside to cool slightly.
In a large bowl using an electric hand-whisk, whisk together the cocoa paste, butter and sugar for 1-2minutes until thick and well combined. Then add the eggs one at a time, mixing well between each addition. Gently mix in the flour and baking powder until just combined. Then gently stir in the milk until combined. Pour into the lined tin and bake for 30 minutes until a knife inserted into the centre of the cake comes out clean. Leave to cool in the tin for 10 minutes, then remove and leave on a wire rack to cool completely.
For the buttercream: Place the butter into a large bowl, and using an electric hand-whisk, whisk butter for 2-3 minutes until very soft and visibly lightened. Then add icing sugar in a few increments, whisking with each new addition until well combined. Keep whipping once all the icing sugar has been added for a few more minutes. Finally, add the vanilla and milk and whisk again until combined. Then, separate the buttercream into three bowls and add food colouring to each.
To assemble: Using a heart cookie cutter, carefully stamp out 6 hearts from the cooled chocolate cake. Then place them onto a baking tray and place into the freezer to chill for 10 minutes. Then place one heart cake onto a cake board or plate (spreading a little buttercream underneath to help it stick if desired), and spread a generous amount of one coloured buttercream on top. Then place another heart cake on top and press down slightly to sandwich. Spread more of the buttercream all over the top and sides of the heart cakes, and use a cake scraper or offset spatula to smooth it out. This is your crumb coat. Then chill in the fridge for 5-10 minutes until firm.
Spread another coat of buttercream all over the tops and sides of the heart cake, and smooth it out again until you're happy with the finish. Place the finished cake into the fridge to set, and repeat the process with the remaining four hearts and buttercreams.
Finally, combine the icing sugar and water in a small bowl until you have a very thick paste. Place this into a piping bag and snip off a small hole at the end. Using this, pipe different sayings onto the tops of each heart cake.