This cake is quite a production; and, since there are a number of good bakeries in downtown Toronto, it is not surprising that I haven't made it in years.   A Patachou cake makes an excellent—albeit expensive—birthday cake; so why bake your own?   However, twenty-five years ago I did a lot more baking; and, despite the time and complications, I made this recipe several times.   It was very popular.   In fact, one year, my father especially requested that I make it as his birthday cake, rather than getting him a boughten one.
        The use of ground nuts to replace flour is common in European cake recipes.   Walnuts, almonds, and hazelnuts are all used for this purpose, each imparting its own distinctive flavour.   Walnuts are commonly used for cakes with dark chocolate icing, since they have a flavour strong enough to compete.   The secret to the success of this particular version, however, is its filling.   I kept getting asked what the secret ingredient is.   The secret, of course, is that there is no secret ingredient:   no coffee, spice, or liqueur to add that extra bite.   The secret is in what you leave out.

Ingredients

Cake:
6 eggs
5/8 cup white sugar
½ tsp cinnamon
1¾ cups ground walnuts
½ tbsp fine dry breadcrumbs
½ tbsp flour

Filling:
1 cup whipping cream
2 tbsp cocoa

Icing & Decoration:
dark chocolate icing
¼ cup walnut nibbits
8 undamaged walnut halves (optional)

Directions

Grinding the Walnuts
If you are unable to get ready-ground walnuts, grind them in a food processor.   You will then have to sift them, since it is important for the texture of the cake that the ground walnuts be very fine.   As the walnuts are ground, the oil is expressed, so there is a tendency for them to cake.   When this starts to happen, add the flour and breadcrumbs to soak up the oil.

Mixing the Batter

Separate the eggs.   Beat the egg whites until they are stiff.

Beat the egg yolks in a separate bowl until they are thick and light in colour.

Mix the ground walnuts and cinnamon with the egg yolks.

Fold in the egg whites.

Baking the Cake

Heavily butter an 8"-inch springform cake pan, and dust it with flour.

Pour in the cake batter.

Bake at 300°F for one-and-a-half hours, or until the cake is done when checked with a cake tester.

Invert the cake pan over a wire rack, and leave the cake to cool.

Cutting the Cake Layers
Remove the cake from the springform pan.   Use a long sharp knife, such as a carving knife, to slice the cake through horizontally twice to produce three layers.

Making the Filling
Whip the cream until it is starting to thicken.   Add the cocoa, and mix it in.  Then continue to beat the cream until it is very thick.

Assembling the Cake
You will probably find it easier to assemble and decorate the cake on the plate you intend to serve it on.   It's all too easy to smudge the icing—or even drop the cake!—if you try to move it after it's been decorated.

Use a blunt knife or spreader to put half the whipped cream on the bottom layer of cake.   Be careful to spread the filling barely to the edge of the cake:   the weight of the layers above will somewhat flatten it, and you don't want it to bulge out and interfere with icing the cake.
        Top the first layer of filling with the second layer of cake.   Spread the rest of the whipped cream on top.   Then put the final layer of cake on top.   Slight nudging is possible if the layers are a bit askew:   you want the cake to look reasonably straight-sided.

Icing and Decorating the Cake
Spread the sides and top of the cake evenly with dark chocolate icing.   Pat walnut nibbits over the sides of the cake.   Decorate the top of the cake with rosettes of icing or filling.   If desired, undamaged walnut halves can be placed either on top of the rosettes or in alternation with them.

Makes one large cake.