Preheat oven to 350 degrees F. Grease and line with parchment paper three 8 inch round baking tins.
Melt the chocolate in a saucepan set over a simmering pan of water. Stir until it is melted and remove from heat to cool slightly.
Whisk together the water, milk and vanilla. Set aside
In a medium bowl, sift together the flour, cocoa powder, baking powder and salt.
In the bowl of your electric mixer, mix the butter and sugar until light and fluffy. This will take a few minutes. Add in the eggs one a time until well incorporated.
Add the flour and milk to the batter in three additions, starting and ending with flour. Stir until just combined. Pour batter into the three prepared pans. Smooth the tops. Bake for approximately 25-30 minutes. Remove from oven and leave to cool on a baking rack.
Make the ganache by heating the cream and butter in a saucepan over medium heat. Once it comes to a slow boil, remove from heat and add the chocolate. Stir until all the chocolate has melted and leave to cool
In a very cold bowl with cold beaters, mix the cream on high speed until it is light and fluffy. Don't over mix or it will likely curdle.
Once the cake has completely cooled you can put it together. Put the bottom layer on the serving dish you are using. Spread over the whipped cream and then top with the next layer. Then put on the top layer. Pour over your ganache in the center of the cake. This will ensure it drizzles quite evenly over the cake and drips down the sides. Leave the cake to set for a few hours before cutting.
This cake can be stored in an airtight container in the fridge for about 3 days.