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1
Heat the oven to 170 °C. Butter and flour a 24 cm springform tin. Melt the dark chocolate and set aside to cool to body temperature (approximately 37 °C). The chocolate-butter emulsion cannot accept hot chocolate; the fat will split if the chocolate is added warm. When a drop pressed to the inner wrist feels neither hot nor cold, the chocolate is ready to use.
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2
Cream the softened butter with the icing sugar until pale and light. Beat in the egg yolks one at a time, incorporating each fully before adding the next. Fold in the cooled melted chocolate. The mixture should be smooth, uniform in colour, and hold a slow ribbon when lifted on the spatula.
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3
Whip the egg whites, adding the caster sugar gradually, until they hold firm, glossy peaks. Fold one-third of the whites into the chocolate mixture to loosen it, then fold in the remaining whites in two additions, alternating with the sifted flour. Work with a light hand; the batter must remain aerated. It is ready when no white streaks remain and the batter falls from the spatula in a slow, heavy ribbon.
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4
Pour the batter into the prepared tin and smooth the surface. Bake at 170 °C for approximately 50 minutes. This is where the dish lives or dies: the sponge must be dense but moist — over-baking produces a dry crumb that destroys the cake's character. At 50 minutes, insert a skewer into the centre; it should emerge with moist crumbs clinging to it, not wet batter. If batter clings, return the tin for five-minute intervals. When the skewer reads moist crumbs, remove from the oven immediately.
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5
Turn the torte onto a wire rack and cool completely. The torte must be entirely cold before any glaze is applied — a warm torte will melt the fondant and allow it to seep into the crumb. The surface should feel cool to a sustained palm-press, with no residual warmth at the centre.
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6
Warm the apricot jam and pass it through a fine sieve, pressing to extract all liquid; discard any solids. The strained jam must be smooth enough to coat a spoon in a thin, unbroken layer. Following the Hotel Sacher Originalrezept, apply the strained jam in a single even coat over the top and sides of the cold torte only (do not split the sponge). The jam surface should be glossy and perfectly smooth — any bump or lump will telegraph through the fondant glaze.
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7
Combine the fondant and water in a small saucepan over very low heat, stirring until fluid. Stir in the melted dark chocolate. Monitor the temperature with a thermometer; the glaze must reach and hold 37–40 °C. Below this range it will set before flowing smooth; above it, it will run off the edges before setting. When the thermometer reads within this window and the glaze coats the back of a spoon in a thin, mirror-bright film, it is ready to pour.
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8
Set the jam-coated torte on a wire rack over a tray. Pour the fondant glaze in a single continuous motion from the centre outward, allowing it to flow over the sides under its own weight. Do not spread with a palette knife — any working of the surface will dull the gloss. The glaze is set when it no longer moves and the surface reflects light as a single unbroken mirror. Allow to set fully at room temperature; do not refrigerate to accelerate setting.
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9
Rest the glazed torte at room temperature until required. Cold from the refrigerator, the fondant hardens and the sponge compresses; the torte must be served at room temperature (the Hotel Sacher specifies 18 °C) for the sponge to read moist and the glaze to have its correct yielding texture. Serve each slice with a roulade of unsweetened whipped cream — a functional foil to the richness of the chocolate, not a garnish.