Rigó Jancsi Chocolate Cake

Preheat oven to 180°C (350°F). Grease and flour a 23x33 cm (9x13 inch) baking pan. Separate eggs. Beat egg yolks with half the sugar until pale and fluffy. In another bowl, beat egg whites with the remaining sugar until stiff peaks form. Gently fold the yolk mixture into the whites. Sift flour and cocoa powder together, then gently fold into the egg mixture until just combined. Pour into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted comes out clean. Let cool completely on a wire rack.

Chop dark chocolate. Heat heavy cream in a saucepan until simmering, then remove from heat. Add chopped chocolate and let sit for 5 minutes, then stir until smooth. Add butter and optional rum, stir until melted and combined. Let the mixture cool to room temperature, then chill in the refrigerator for at least 2-3 hours, or until very cold and thick.

Once the chocolate cream is thoroughly chilled, whip it with an electric mixer until light and fluffy. Be careful not to over-whip, or it might curdle. Slice the cooled sponge cake horizontally into two equal layers. Place one layer on a serving platter. Spread the whipped chocolate cream evenly over this layer. Place the second sponge layer on top.

Melt dark chocolate and butter with water over a double boiler or in the microwave until smooth. Pour the warm glaze over the top of the cake, spreading it evenly.

Refrigerate the cake for at least 1-2 hours to allow the cream and glaze to set. Before serving, you can score the top of the glaze into squares for easier cutting.