All ingredients for making these extraordinary cakes can be found in ordinary grocery stores.
I use unbleached all-purpose flour, eggs labeled as large by American standards (or medium by European standards), bittersweet chocolate, and unsalted butter for all my cakes. Never use salted butter to make these cakes.
Cake flour has less gluten than all-purpose flour and produces a more velvety, lighter cake structure, but the difference in structure is not readily noticeable in cakes presented in this book since they all contain very little or no flour. Nevertheless, all sponge (whole-egg based) cakes in this book can be made with cake flour or cornstarch for gluten-free versions. Make cake flour from all-purpose flour by substituting two tablespoons of flour of each cup of flour by two tablespoons of cornstarch.
Confectioners’ sugar is not the same as powdered or superfine sugar as it contains a small amount of cornstarch, which prevents it from hardening. Both confectioners’ and superfine sugar can be used in meringues. Meringue batter can also be made with ordinary granulated sugar, but it may take too much time for the sugar to dissolve, depending on the size of the sugar granules. Therefore, it may be simpler to first make superfine sugar by grinding the sugar in an electric grinder or food processor and then proceeding to make meringue batter.
Vanilla flavor can be added to cakes in three basic ways. The old-fashioned way to add vanilla flavor to milk is to soak a dried vanilla bean in scalded milk for half an hour. The vanilla bean can then be taken out, dried, and reused later. As a bonus, the whole house will acquire a heavenly scent of vanilla after this procedure. Another way to add vanilla flavor is to add vanilla sugar (commonly available in Europe in 10 g packages). The third way is to add vanilla extract. My preference is in the order given, with vanilla extract being my last choice because it is the least pure among vanilla additives, containing other substances, including alcohol.
As always, the fresher and finer the ingredients, the better the results! This is particularly true for walnuts (or nuts of any kind), since they change taste if stale and can certainly ruin the cake if rancid. Nuts should be sampled before use, as a precaution, and not used if they fail the taste test. They should be kept frozen and used straight from the freezer, without thawing. Butter also degrades with aging and should be kept frozen, but unlike nuts, butter should be thawed and brought to room temperature before use.