Prepare the Cake Batter

1. Preheat Oven and Prepare Pans: Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and line the bottom with parchment paper for easy removal after baking.
2. Melt the Chocolate: In a heatproof bowl, melt the dark chocolate over a double boiler, stirring occasionally until smooth. Remove from heat and let it cool slightly.
3. Cream Butter and Sugar: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. This should take about 3-5 minutes with an electric mixer.
4. Add Egg Yolks and Vanilla: Beat in the egg yolks, one at a time, ensuring each is fully incorporated before adding the next. Stir in the vanilla extract.
5. Combine Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, and salt. Gradually fold these dry ingredients into the butter mixture, alternating with the melted chocolate. Start and end with the dry ingredients, mixing gently to avoid deflating the batter.
6. Whisk Egg Whites: In a clean bowl, beat the egg whites until stiff peaks form. This will help give the cake its light and airy texture.
7. Fold in the Egg Whites: Gently fold the whipped egg whites into the chocolate batter in batches. Be careful not to overmix, as you want to maintain the airy texture of the egg whites.
8. Bake the Cake: Pour the batter into the prepared pan and smooth the top. Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Prepare the Apricot Glaze

1. Heat the Jam: In a small saucepan, combine the apricot jam and water. Heat over medium-low heat, stirring constantly, until the jam becomes thin and smooth.
2. Strain the Glaze (Optional): If you prefer a smoother glaze, you can strain the jam to remove any bits of fruit, ensuring the glaze is silky and glossy.

Assemble the Sacher Torte

1. Slice the Cake: Once the cake has cooled completely, carefully slice it in half horizontally to create two even layers.
2. Spread the Apricot Jam: Warm the apricot glaze slightly if it has thickened, then spread a generous amount over the bottom layer of the cake. Place the top layer on top and gently press down to ensure the layers stick together.
3. Apply the Chocolate Coating: In a heatproof bowl, melt the dark chocolate, butter, and heavy cream together until smooth. Pour the melted chocolate over the top of the cake, allowing it to drip down the sides for that signature glossy finish. Use a spatula to spread the chocolate evenly across the top and sides.