Rigó Jancsi (Chocolate Mousse Squares)

Sponge. Heat the oven to 350°F (180°C) and line two 25 × 35 cm (10 × 14 inch) baking sheets. Beat the egg whites with the salt to stiff peaks, adding the granulated sugar (120 g½ cup + 2 tbsp) gradually. Fold in the yolks. Sift the flour (30 g¼ cup) and cocoa powder (50 g½ cup + 2 tbsp) together and fold them in gently.

Divide between the sheets and spread thin and even. Bake 10 to 12 minutes, just until set. Cool.

Chocolate cream. Heat 150 ml½ cup + 2 tbsp of the heavy cream (400 ml1⅔ cups) to a simmer, pour it over the dark chocolate (200 g) and stir until smooth. Cool to room temperature. Whip the rest of the cream to soft peaks, then fold in the chocolate mixture in stages until airy and mousse-like. Add the rum (1 tbsp), if using.

Assemble. Lay one sponge on a tray and spread all the chocolate cream over it, 3 to 4 cm (1¼ to 1½ inches) thick. Lay the second sponge on top and press gently.

Glaze. Melt the dark chocolate (150 g) with the butter (50 g¼ cup) and oil (14 g1 tbsp), cool slightly, and spread it over the top. Refrigerate at least 4 hours.

Cut into squares with a hot knife, wiping it between cuts.