Step 1

Preheat oven to 180°C. Spray two 2cm-deep, 22.5 x 33cm Swiss roll pans with oil. Line with non-stick baking paper.

Step 2

Use an electric beater to beat the eggs, sugar and vanilla in a bowl until thick and pale. Combine flour, cocoa powder and baking powder in a bowl. Sift one-third of the flour mixture over the egg mixture. Use a large metal spoon to fold until just combined. Repeat, in 2 more batches, with the remaining flour mixture. Divide evenly between prepared pans and smooth the surfaces. Bake for 15 minutes or until the cakes spring back when lightly touched. Set aside in the pans on a wire rack to cool completely.

Step 3

Meanwhile, to make the mousse, use an electric beater to beat the egg yolks and icing sugar in a bowl until thick and pale. Beat in the chocolate and Frangelico. Whisk the cream in a bowl until soft peaks form. Use a large metal spoon to gently fold the cream into the chocolate mixture. Use an electric beater to beat the egg whites in a clean, dry bowl until soft peaks form. Fold one-third of the egg white into the chocolate mixture. Repeat, in 2 more batches, with remaining egg white. Cover surface with plastic wrap.

Step 4

Spray a 7.5cm-deep, 8.5 x 17cm loaf pan with oil and line with plastic wrap. Turn the cakes onto a sheet of non-stick baking paper. Remove the top sheets of paper. Line the base and sides of the prepared loaf pan with cake, cutting to fit. Reserve excess cake. Cut two 8.5 x 17cm rectangles from the reserved cake.

Step 5

Spoon half the mousse over the cake base and smooth the surface. Top with 1 cake rectangle, pressing down gently. Spoon over the remaining mousse and smooth the surface. Top with the remaining cake rectangle and press down gently. Wrap in plastic wrap and place in the fridge for 4-6 hours or overnight to set.

Step 6

Stir the chocolate and butter in a saucepan over low heat until chocolate melts and the mixture is smooth. Turn the cake onto a serving platter and pour over the chocolate sauce.