Butter the insides of the tin, and line the bottom with parchment.
Put the chocolate into a bowl set over simmering water - make sure the bowl isn’t touching the water. Once the chocolate has melted, take it off the heat and add the butter, then whisk it through until smooth and glossy.
Put the 4 egg whites into a large mixing bowl and use an electric hand whisk to whisk them to soft peaks. Add in 1/2 cups sugar, then carry on whisking until it reaches firm peaks.
Put the 4 egg yolks into a separate bowl. Add in the 2 whole eggs, and the other 1/2 cup of sugar. Whisk until pale and fluffy. Tip the chocolate mixture into the bowl, add the espresso powder and whisk again until mixed through.
Fold the egg whites into the chocolate mix, a few big spoonfuls at a time. Once the egg whites are completely mixed in, scrape the cake mixture into the prepared tin and smooth it over.
Bake for 35 -40 minutes, until just set, then cool completely on a wire rack, still in the tin.
Once fully cool, carefully remove from the tin and put onto a serving plate or stand.
Vanilla Cream Topping
Decorate the cake: Put the cream and vanilla into a large mixing bowl and whip to soft peaks.
Sieve in the icing sugar and whisk again, stopping before the cream reaches firm peaks but when it’s firm enough to hold its shape.
Create the chocolate birds: Cut a strip of parchment paper the right size to fit inside your book. Open the book about halfway through and let it sit open on the counter. Fold the paper in half to create a neat seam along the middle, then slot that into the spine of the open book.
Use a fine paintbrush to paint a short line of chocolate, along the fold in the paper, for the body of the bird. Paint one brush stroke either side, to create wings.
Repeat until you have 5 or 6 birds, the leave to set in the fridge.
Once set, carefully remove the birds from the paper.
Spoon the cream into the crater of the cake, piling it up high.
Dust with cocoa powder then carefully place the chocolate birds on top. The cake is ready to serve straight away.