Preheat oven to 350°F (175°C). Grease a 9-inch round cake pan, line with parchment paper, then grease and flour the parchment.
Melt the chocolate in a heatproof bowl set over a pan of simmering water. Set aside to cool slightly.
In a large bowl, beat the butter and ¼ cup of the sugar until light and creamy. Add egg yolks one at a time, beating well after each addition. Stir in the cooled chocolate and vanilla extract.
In a separate bowl, beat egg whites with a pinch of salt until foamy. Gradually add the remaining ½ cup sugar and beat until stiff peaks form.
Gently fold a third of the egg whites into the chocolate mixture to lighten it. Sift the flour over the mixture and fold in, followed by the remaining egg whites. Fold gently to maintain volume.
Pour batter into the prepared pan and smooth the top. Bake for 40-45 minutes until a toothpick inserted in the center comes out with a few moist crumbs.
Cool in the pan for 10 minutes, then invert onto a wire rack to cool completely.
When cool, cut the cake horizontally into two layers.
Heat the apricot preserves until melted and smooth. If using, stir in the rum or liqueur. Strain if lumpy.
Brush the top of the bottom layer with preserves, then place the second layer on top. Brush the entire cake with the remaining apricot glaze. Let set for 30 minutes.
For the chocolate glaze, place chocolate in a bowl. Heat cream until just about to simmer, then pour over chocolate. Add corn syrup and let sit for 1 minute, then stir until smooth.
Place cake on a rack over a baking sheet. Pour glaze over cake, ensuring even coverage. Allow glaze to set for at least 1 hour.
Traditionally served with a dollop of unsweetened whipped cream.