Prep the pan and oven. Heat oven to 350°F. Grease a 9-inch round cake pan (at least 2 inches deep) and line the bottom with parchment. Lightly dust with cocoa powder.
Pan note: If you use a springform, wrap the outside bottom with a snug layer of foil to prevent leaks.
Melt chocolate and butter. In a small saucepan over low heat (or a microwave in short bursts), melt the butter and 4 oz chopped chocolate until smooth. Set aside to cool slightly.
Whip yolks. In a large bowl, beat egg yolks with 1/2 cup of the sugar until pale and thick, about 2 to 3 minutes. Beat in vanilla. Slowly mix in the cooled chocolate-butter mixture.
Whip whites. In a clean bowl, beat egg whites with the salt until foamy. Gradually add the remaining 1/4 cup sugar and whip to glossy medium-stiff peaks. The tip should stand but still look creamy, not dry.
Fold. Sift flour and cocoa over the chocolate yolk mixture and fold gently. Fold in one-third of the egg whites to lighten, then fold in the remaining whites in two additions.
Tip: Use a large spatula and a light hand. Sweep down the side, scoop from the bottom, and rotate the bowl as you go. Stop as soon as you do not see white streaks.
Bake. Scrape batter into the pan and smooth the top. Bake 28 to 35 minutes, until the center springs back and a toothpick comes out with a few moist crumbs. Cool in the pan 10 minutes, then turn out onto a rack and cool completely.
Make optional simple syrup. In a small saucepan, simmer 2 tbsp water and 2 tbsp sugar just until dissolved. Cool.
Slice and fill. Make sure the cake is fully cool before slicing. Level the cake if needed. Slice horizontally into 2 layers.
Warm the apricot jam in a saucepan until loose, adding 1 tbsp water if needed. Strain for a smooth finish. Brush the cut sides lightly with simple syrup if using, then spread a thin, even layer of jam on the bottom layer. Place the top layer on.
Crumb coat with jam. Spread a very thin layer of warm jam over the top and sides of the cake. This is a classic step that helps the glaze look neat and polished. Let it set until tacky and no longer wet to the touch, about 10 to 15 minutes.
Make the glaze. Bring sugar, water, and corn syrup (if using) to a gentle simmer for 3 to 4 minutes, just until slightly thickened. Visual cue: it should lightly coat a spoon, not reduce into a thick syrup.
Remove from heat, add chopped chocolate and salt, and stir until smooth. Let cool 5 to 8 minutes so it thickens slightly but still pours. Strain if you want it extra silky.
Glaze the cake. Set cake on a wire rack over a rimmed baking sheet. Pour glaze over the center and let it flow to the edges. Use an offset spatula only if needed, and keep it minimal to avoid marks.
Let set 1 to 2 hours at room temperature (or 30 minutes in the fridge, then bring back to room temp before serving). For a cleaner finish, you can trim any long drips after the glaze sets.
Transfer, slice, and serve. To move it cleanly, slide a long offset spatula under the cake and transfer to a serving plate. Use a warm knife for clean slices. Traditionally, serve with lightly sweetened whipped cream on the side.
