1) Preheat the oven to 350°F. Spray one 9" by 2" round cake pan with cooking spray and line with parchment. Spritz the parchment lightly as well. If your pan isn't at least 2" deep, use two pans instead of one.
2) Over low heat or in the microwave melt the chocolate slowly, stirring well.
3) In a small bowl, lightly beat the 8 egg yolks. Add the melted chocolate, melted butter, and vanilla. Blend until smooth and satiny, with no lumps or unincorporated yolks.
4) In the bowl of a stand mixer, beat the 8 egg whites with a pinch of salt until they begin to foam. Slowly add the sugar, then beat on high speed until the whites hold a stiff peak but are still glossy.
5) Using a wide rubber spatula, mix about 1/3 of the egg whites into the chocolate/yolk mixture to lighten it. Now, pour the lightened chocolate mixture over the rest of the whites in the bowl. Fold gently, using about 20 to 30 strokes.
6) Sprinkle the cake flour over the chocolate batter and continue to fold gently until there are no traces of egg white remaining.
7) Pour the batter into the pan(s). Bake until the cake is puffed and dry looking on top, and a cake tester inserted into the center comes out clean: 40 to 45 minutes for a single pan, 20 to 25 minutes for two pans. Remove from the oven and cool in the pan(s) for 10 minutes before turning out onto a wire rack to cool completely. Be sure to peel off the parchment circle while the cake is still warm.
8) While the cake is cooling, strain the apricot jam through a fine sieve to remove any bits of fruit and make a smooth filling.
9) Filling the cake: If you used one cake pan, you will need to split the layer before filling. Use a long, sharp serrated knife to split the cake into two even layers. If you used two pans, simply spread the apricot jam between the layers, leaving 1/2 inch around the border so that the jam does not squeeze out over the sides of the cake.
10) For the chocolate water glaze: Place the filled cake on a wire rack over a parchment lined baking sheet. Melt the chocolate in a deep, heat safe bowl in the microwave. As soon as it is melted and no lumps remain, vigorously stir in the boiling water. If you go too slowly the chocolate may seize, so incorporate the two ingredients quickly.
11) As soon as the glaze is smooth, immediately pour over the cake. The excess glaze will drip off of the cake onto the parchment paper. You can scoop up the excess glaze to cover any bare spots on the cake. Use a flexible spatula to help spread the glaze on the top and sides of the cake, but do not overwork the glaze or it will not remain smooth and sleek.
12) Allow the glaze to set up at room temperature for a few hours before serving. This cake is best served the day it's made. Store any leftovers at room temperature for 1 day.
Yield: one 9" cake.