Preheat the oven to 170°C (340°F). Grease and line a 23 cm (9 in) springform pan with parchment.
Melt the chopped Dark Chocolate and Unsalted Butter together in a heatproof bowl over gently simmering water (bain-marie), stirring until smooth; remove from heat and let cool slightly.
In a large bowl, whisk the Eggs with the Caster Sugar and Vanilla Extract until pale, thick and ribbon-like (about 8–10 minutes with an electric mixer).
Fold the melted chocolate-butter mixture into the egg mixture until evenly combined, then gently fold in the sifted Plain Flour and a pinch of Fine Salt until just blended — avoid overmixing to keep the cake tender.
Pour the batter into the prepared pan, smooth the top, and bake for 45–55 minutes, or until a skewer inserted into the center comes out with a few moist crumbs. Cool the cake in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Slice the cooled cake horizontally into two layers. Warm the Apricot Jam with the Dark Rum and a tablespoon of Water in a small saucepan, stirring until smooth and slightly loose — this thins the jam for easy spreading. Brush the jam mixture over both cut surfaces and reassemble the cake.
For the glaze, heat the Heavy Cream with a tablespoon of Caster Sugar until just simmering. Pour the hot cream over additional chopped Dark Chocolate (reserved for the glaze) and whisk until glossy and smooth; stir in a small knob of Unsalted Butter if you like extra shine.
Place the assembled cake on a wire rack over a tray and pour the warm glaze over the top, using a spatula to encourage an even coating down the sides. Allow the glaze to set at room temperature.
When ready to serve, dust lightly with Icing Sugar if desired and slice with a hot, dry knife for clean cuts. Enjoy with a dollop of whipped cream or a cup of strong coffee.
