1. Preheat and prep: Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan (preferably springform) with butter and line the bottom with parchment paper. Grease the parchment as well and lightly dust with flour, tapping out excess.
2. Melt the chocolate: Place chopped chocolate in a heatproof bowl set over a pan of barely simmering water (double boiler method), stirring occasionally until smooth and completely melted. Alternatively, microwave in 20-second intervals, stirring between each. Remove from heat and let cool slightly, about 5 minutes.
3. Cream butter and sugar: In a large mixing bowl, beat the room-temperature butter with an electric mixer on medium speed for 1-2 minutes until creamy. Add ½ cup (100g) of the sugar and beat for another 3-4 minutes until light, fluffy, and nearly white in color. This step incorporates air that helps the cake rise.
4. Add egg yolks: Add the egg yolks one at a time, beating well after each addition. The mixture should be smooth and glossy. Beat in the vanilla extract.
5. Incorporate chocolate: Pour in the slightly cooled melted chocolate and beat on low speed until completely combined and uniform in color. The mixture will be thick and glossy.
6. Add flour: Sift the flour and salt together, then gently fold it into the chocolate mixture using a rubber spatula. Fold just until no white streaks remain—don’t overmix! Set this bowl aside.
7. Whip egg whites: In a completely clean, grease-free bowl, beat the egg whites with the cream of tartar on medium speed until foamy. Gradually add the remaining ¼ cup (50g) sugar, one tablespoon at a time, beating continuously. Increase speed to medium-high and beat until stiff, glossy peaks form. When you lift the beaters, the peaks should stand straight up without drooping. This takes about 4-5 minutes total.
8. Fold egg whites: Take about one-third of the whipped egg whites and fold them gently into the chocolate batter using a rubber spatula. Use a light hand and a folding motion (cutting down through the center, across the bottom, and up the side). This lightens the batter. Now fold in the remaining egg whites in two additions, being careful not to deflate them. The batter should be airy and mousse-like with no white streaks remaining.
9. Bake: Pour the batter into the prepared pan and smooth the top gently. Tap the pan lightly on the counter twice to release any large air bubbles. Bake for 35-40 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter). The top should spring back lightly when touched.
10. Cool: Let the cake cool in the pan for 15 minutes, then run a knife around the edges to loosen. If using a springform pan, release the sides. Carefully invert the cake onto a wire rack, remove the pan bottom and parchment, then flip right-side up onto another rack. Cool completely, about 1-2 hours.

Preparing the Apricot Glaze:

11. Warm and strain: In a small saucepan, combine apricot preserves and water. Heat over medium-low heat, stirring, until the preserves melt and become liquid. If your preserves are chunky, press the mixture through a fine-mesh strainer into a bowl to remove any fruit pieces. Return to the saucepan and keep warm.
12. Split the cake (optional but traditional): Using a long serrated knife, carefully slice the cooled cake horizontally into two even layers. This is optional—some bakers prefer a single layer cake, which is also acceptable.
13. Brush with glaze: Place the bottom layer (or whole cake if not splitting) on a wire rack set over a baking sheet or parchment paper. Brush the top generously with warm apricot glaze. If you split the cake, place the second layer on top and brush the top and sides with the remaining apricot glaze, creating a thin, even coating all around. The apricot layer seals in moisture and provides a smooth surface for the chocolate glaze. Let this set for 15-20 minutes.

Making the Chocolate Glaze:

14. Prepare the ganache: Place the finely chopped dark chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream until it just begins to simmer (small bubbles around the edges—don’t let it boil). Immediately pour the hot cream over the chocolate. Let sit for 1 minute, then add the corn syrup and butter.
15. Create smooth glaze: Whisk gently from the center outward until the mixture is completely smooth, glossy, and uniform. Let it cool for 5-10 minutes, stirring occasionally. The glaze should be slightly warm (about 90-95°F) and have a thick but pourable consistency—like heavy cream. If it’s too thick, warm it gently; if too thin, let it cool a bit more.
16. Glaze the cake: Ensure your apricot-coated cake is on a wire rack over a pan to catch drips. Pour the chocolate glaze over the center of the cake, letting it flow naturally over the top and down the sides. Use an offset spatula to gently encourage the glaze to cover any bare spots, working quickly as the glaze begins to set. Aim for smooth, even coverage. Don’t overwork it—a few drips add rustic charm!
17. Set the glaze: Let the glazed cake sit at room temperature for at least 1 hour, or until the chocolate glaze is completely set and no longer tacky to a light touch. For best results, refrigerate for 30 minutes to set the glaze quickly, then bring back to room temperature before serving.
18. Serve with cream: When ready to serve, whip the heavy cream with powdered sugar (if using) and vanilla until soft peaks form. Slice the Sachertorte with a sharp knife (wipe clean between cuts) and serve each slice with a generous dollop of unsweetened or lightly sweetened whipped cream on the side, as is traditional in Vienna.