1. Melt the chocolate in a bain-marie or a bowl placed over a pan with hot water.
2. Preheat the oven to 180°C/350F/gas 4. Butter and flour a 23cm cake tin, shake off excess of flour and place a disc of parchment at the bottom of the tin.
3. Beat the butter with the icing sugar and vanilla until light and fluffy. Add the egg yolks, one by one, beating well in between. Whip up the egg whites with a pinch of salt until soft peaks form and start to gradually add the caster sugar. Beat until stiff peaks form.
4. Stir the melted chocolate into the egg yolk mix and fold in the egg whites alternately with the flour.
5. Pour the batter into the tin, flatten the middle (it will rise into a dome) and bake for about 50 minutes. Check that the skewer inserted in the middle comes out clean.
6. Take the tin out of the oven and immediately turn out onto a wire rack lined with parchment, leave the tin on. After 25 minutes turn it right side up and cool completely in the tin.
7. Heat up the apricot jam to soften it. Cut the cake with a very sharp knife horizontally, spread the jam over the base and top with the other half. Spread the rest of the jam on top and over the sides, leave to soak in.
8. For the glaze, break the chocolate into small pieces. Heat up the water with the sugar, leave to cool down until just warm. Add the chocolate and stir to melt.
9. Pour the glaze quickly over the cake and immediately spread it out and smooth it over the surface, with a palette knife. Leave the cake to dry at room temperature. Serve with whipped cream. Don’t store the Sacher torte in the fridge, as it will sweat.