Preheat the oven to 350°F / 175°C. Grease a 9in / 23cm round cake pan at least 2in / 5cm deep and line it with parchment paper. Melt half of 2 cups of bittersweet chocolate wafers, keeping the rest for the glaze.
Separate 8 large eggs. Lightly beat the yolks, then stir in the melted chocolate, 8 tablespoons of melted unsalted butter, and 1 teaspoon of vanilla extract.
Whip the egg whites with ⅛ teaspoon of salt, then beat in ¾ cup of superfine sugar until they hold stiff, glossy peaks.
Stir some of the whites into the chocolate mixture, then fold in the rest. Gently fold in 1 cup of cake flour.
Spread in the pan and bake for 40-45 minutes, until the top is puffy and dry and a cake tester comes out clean. Cool in the pan for 10 minutes, then turn out onto a rack to cool completely.
Push ½ cup of apricot jam through a fine sieve. Split the cake into two even layers with a long serrated knife, spread the jam over the bottom layer, and replace the top.
For the glaze, cook the remaining chocolate with ½ cup of boiling water and 1 cup of granulated sugar over medium-low heat for 4-5 minutes, until smooth and slightly thickened.
Set the cake on a rack over a parchment-lined baking sheet and pour the glaze over it. Leave at room temperature until the glaze sets.
