iSacher Torte

Heat the oven to 350°F (175°C). Grease a 9-inch (23-cm) springform pan with a little unsalted butter and line the bottom with parchment.

Set up a saucepan with 1 inch of simmering water and place a heatproof bowl over it (or use short microwave bursts). Melt about half of the chopped bittersweet chocolate until smooth, then remove from heat and let cool to lukewarm.

In a large bowl, beat the remaining butter with the confectioners' sugar until very light and fluffy, 3–4 minutes. Separate the large eggs, reserving whites in a clean bowl. Beat the yolks into the butter mixture one at a time, then mix in the vanilla extract. Blend in the melted chocolate until smooth.

With a clean whisk or mixer, beat the whites with the fine salt on medium speed to soft peaks, 1–2 minutes. Gradually add 0.5 cup of the granulated sugar and beat to glossy medium-stiff peaks, 1–2 minutes more.

Stir a large spoonful of the meringue into the chocolate base to loosen. Sprinkle the all-purpose flour over the batter and fold it in gently, alternating with the remaining meringue in 2–3 additions, just until no streaks remain.

Scrape the batter into the prepared pan, smooth the top, and bake until the center springs back and a toothpick comes out with a few moist crumbs, 45–55 minutes. Cool in the pan 10 minutes, then invert onto a rack, peel off parchment, and cool completely, 1.5–2 hours.

Level the top if domed, then split the cake horizontally into two even layers with a long serrated knife.

Warm the apricot jam in a small saucepan until fluid, then press through a sieve. Spread a thin layer over the cut sides, sandwich the cake, and brush a very thin coat over the top and sides. Let the jam set until tacky, 20–30 minutes.

For the glaze, combine the water with the remaining sugar in a small saucepan. Bring to a boil over medium heat, then simmer until slightly syrupy with large, clear bubbles, about 5 minutes. Remove from heat, let the bubbles subside for 1–2 minutes, then add the remaining chocolate and stir gently until smooth, shiny, and fluid; it should fall in a thick ribbon and coat the back of a spoon (about 95–105°F / 35–40°C).

Set the cake on a rack over a tray. Pour the glaze onto the center and around the edge in one continuous pour, nudging quickly with a spatula to cover the sides. Let excess drip off; avoid overworking. Allow to set until the surface is matte and firm to the touch, 45–60 minutes.

Slice with a warm knife, wiping between cuts. Whip the heavy cream just to soft peaks without sugar and serve alongside each slice.