Bake the cake:
Heat the oven to 350 degrees Fahrenheit and grease the bottom of two 8 inch baking pans with round bottoms. In the working bowl of your standing mixer, combine together flour, sugar, baking powder, cocoa powder and salt. In a small mixing bowl, combine two beaten eggs with milk and vegetable oil. With the mixer set on low speed, gradually pour this mixture into the dry ingredients. Keep processing on low until the ingredients are evenly mixed. Then, add hot coffee to the batter and mix well. Now divide the batter equally between two prepared pans and bake for 35 to 40 minutes or until a toothpick inserted into the centre of the cake comes out clean. Remove the cakes from the oven, and allow them to cool completely before cutting each cake into two equal parts.
Make the cocoa-olallieberry buttercream:
Sift the black cocoa powder, mix it into the olallieberry puree and set aside. Take a small saucepan and combine water and sugar in it. Bring the sugar mixture to a boil over medium high heat, and without stirring the syrup, allow it to heat up to 240 degrees Fahrenheit. In the meantime, combine egg whites and cream of tartar in the working bowl of the mixer and whisk on medium low until the egg whites begin to foam. At this stage, increase the speed of the beater to medium high, add the remainder of the sugar and whip the egg whites to soft peaks.
Once the sugar syrup reaches 240 degrees Fahrenheit, pour it into the beaten egg whites in a small stream, whisking continuously. Keep whisking on medium high until the ingredients in the mixing bowl are not warm anymore. Now slowly and gradually add butter to the egg whites and sugar mixture, about ½ tablespoon at a time. Once all the butter has been added, stir in the cocoa and olallieberry mixture. Also add the food colour at this stage (if using). Beat the frosting for another couple of minutes and then use it immediately to layer the baked cake parts.
Make dark chocolate ganache:
Place the chopped chocolate in a medium sized mixing bowl and set aside. Meanwhile, heat the butter and cream in a small saucepan until it is almost simmering, and then pour it over the chopped chocolate. Let it stand for a minute and then whisk the chocolate and cream until smooth. Cool the ganache a bit and then pour it onto the frosted cake. Your Ollieberry chocolate cake is now ready; cool it in the refrigerator for at least half an hour before serving.