Sachertorte is one of the few cakes that have been the subject of court cases. The original recipe remains a guarded trade secret; what is known is that in 1832 an apprentice pastry chef named Franz Sacher made it for a dinner hosted by Austro-Hungarian chancellor Klemens Wenzel Lothar von Metternich. Two layers of chocolate sponge, apricot jam between them, dark glaze on top and whipped cream on the side.

The home version needs no secret. It needs three things done correctly: air in the batter, apricot between the layers, and a glaze that sets with a shine.

For the batter and filling you need 130 grams of butter, 110 grams of icing sugar, 100 grams of caster sugar, 6 eggs, 130 grams of chocolate, 130 grams of flour, a pinch of salt and 4 tablespoons of apricot jam. For the glaze: 130 millilitres of water, 120 grams of sugar and 200 grams of dark chocolate.

The butter and chocolate are melted together, in the microwave or over a bain-marie, until smooth. The eggs are separated. The yolks are beaten with the icing sugar until creamy, then the melted chocolate goes in, followed by the sifted flour with the salt. The six whites are whipped with 100 grams of sugar until genuinely firm and folded in over several additions, slowly, so as not to knock out the air you have just put in. Bake for 30 to 40 minutes at 170 degrees, in a preheated oven.

The whole point is in separating the egg. When yolks and whites are beaten separately, each with its own sugar, the batter takes in twice as much air, and that is the difference between a spongy and a dense crumb. Whoever beat all the eggs together made a different cake.

Cooling is mandatory before cutting. The cooled sponge is cut horizontally in half, the bottom layer is spread with warmed apricot jam, the top goes back on.

The glaze is where most home attempts fail. Water and sugar go on the heat, stirred until the sugar dissolves, then boiled briefly until the syrup thickens slightly. Leave it to cool a little. The chocolate, finely chopped, sits in a bowl and the syrup is added gradually, with constant whisking, until you get a smooth and glossy emulsion. The temperature of the chocolate must not pass 45 degrees, or the shine disappears and never comes back.

The cake goes on a rack over a tray, the glaze is poured into the centre and spread with a spatula across the top and sides. The excess falls through, which is the point of the rack. If you want the full version, keep a little glaze in a piping bag and write the name across the top.

For the chocolate, in both batter and glaze, 70 percent cocoa is recommended. Apricot is traditional, but raspberry, forest fruits, peach or bitter orange marmalade work just as well. Serve it plain, with whipped cream, or with softly whipped cream.