1. Preheat the oven to 350°F (180°C). Line a 9in (23cm) round cake pan with wax paper.

2. Beat the butter and sugar together with an electric mixer until light and fluffy. Beat in the chocolate and vanilla. One at a time, beat in the egg yolks. Fold in the flour.

3. Beat the egg whites until stiff peaks form. Stir one-fourth of the whites into the batter, then fold in the remainder. Spread evenly in the pan.

4. Bake for 45–60 minutes, or until a wooden toothpick inserted in the center comes out clean. Transfer to a wire rack and let cool for 10 minutes. Invert and unmold the cake. Remove the wax paper. Let cool completely.

5. To make the glaze, bring the cream to a simmer in a small saucepan over medium heat. Remove from the heat and add the chocolate and vanilla. Let stand until the chocolate softens, then stir until smooth. Let cool, stirring occasionally, until it thickens slightly.

6. Melt the apricot preserves in a small saucepan over low heat. Strain through a sieve. Slice the cake in half horizontally. Spread the lower layer with some of the apricot mixture. Return the top layer. Spread the remaining apricot mixture over the top of the cake, then use a metal spatula to smooth it evenly over the top and sides. Let cool until set.

7. Place the cake on a wire rack over a baking sheet. Transfer ¼ cup of the glaze to a small bowl. Pour the remaining chocolate glaze over the cake. Use a metal spatula to smooth the glaze over the top and sides of the cake. Patch any unglazed spots with the glaze that runs onto the baking sheet. Refrigerate about 10 minutes, until the glaze sets.

8. If necessary, warm the reserved glaze until fluid. Transfer to a small plastic food storage bag and force into a corner of the bag. Snip off the corner of the bag with kitchen scissors. Squeeze the glaze out of the bag to write "Sacher" over the top of the cake in flowing script. Let cool, and serve with whipped cream.