Method
Prep:1hr30min  ›  Cook:30min  ›  Extra time:4hr cooling  ›  Ready in:6hr 

For the cake:

1. Combine milk and lemon juice; let stand for 10 to 15 minutes, till curdled.
2. Sift together flour, cocoa powder, baking powder, bicarb and salt. Set aside.
3. Beat softened butter with sugar and vanilla sugar, for 3 to 4 minutes, until light and fluffy.
4. Add egg yolks to the butter mixture one at a time, beating after each addition. Gradually add the curdled milk and dry mixture, alternating between the two. After adding all of the curdled milk, add the brewed coffee alternating with the rest of the flour. Beat until the cake mixture reaches a smooth and light consistency, resembling thick frosting.
5. Preheat oven to 180 C / Gas 4. Lightly grease three 23cm cake tins. Cut out circles of parchment and line the bottoms of the tins.
6. Leave about 225ml aside for baking little whoopie pies for decoration. Divide the remaining cake mixture between the prepared tins.
7. Bake cake layers in the preheated oven for 15 to 20 minutes, or until a skewer comes out clean from the centre. Remove from the oven and cool completely.
8. Use the reserved cake mixture to bake small whoopie pies: drop 6 heaping teaspoons of mixture onto a parchment-lined baking tray. Bake for 10 minutes in the preheated oven at 180 C / Gas 4. Remove from oven and cool completely on the tray.

For the frosting:

9. Use a slice of lemon to wipe a large mixing bowl and your whisk, to remove any traces of grease. Add egg whites to the bowl along with sugar. Mix and place the bowl over a pan of gently boiling water (water should not touch the bowl).
10. Using an electric whisk, beat the egg whites constantly until the mixture becomes light, fluffy and significantly increased in volume. When you see it starting to bubble, remove from the heat. Continue beating off of the heat till the meringue becomes white, glossy, sticky and thick. It should not drop from the whisk when ready. The mixture and the bowl should reach room temperature at the end of beating.
11. In a separate bowl, beat butter till light and fluffy. Combine butter with the cooled meringue and beat until well mixed. Place mixture in the fridge for 15 to 30 minutes to stiffen.

To assemble:

12. First frost the whoopie pies: spread frosting on three of the whoopie pies, spreading on the flat side. Top with the three remaining whoopie pies. Cut each whoopie pie in half and set aside.
13. Spread some frosting in the centre of first cooled cake, leaving a 2 to 3cm space around the edge. Using a piping bag, pipe rounds of frosting around the edge. Repeat with the second layer.
14. Assemble the cake by placing the second layer on top of the first, then cover with the third unfrosted layer.

For the ganache:

15. Combine chocolate, cream and butter in a mug and microwave for 1 minute, stopping every 10 to 15 seconds to mix, till smooth.
16. Cover the top of the cake with ganache. Arrange the whoopie pie halves on top of the cake in a decorative fashion. You can also add grated chocolate or sprinkles.
17. Chill cake in the fridge for at least 4 hours or overnight before slicing and serving.

Note

If you just want a plain layer cake, you don't have to bake the whoopie pies and can instead distribute the entire cake mixture between the three tins.