Make the ganache: Melt the chocolate in a double boiler. Heat the heavy cream, then gradually pour it into the chocolate off the heat, stirring until smooth. Chill the mixture in the refrigerator.

For the sponge cake: In a mixing bowl, combine the sugar, vanilla extract, and egg yolks. Beat until the mixture is pale and creamy. Add the flour, cornstarch, orange zest, and baking powder. Mix in the melted, cooled butter. In a separate bowl, beat the egg whites until stiff peaks form. Gently fold the egg whites into the batter in three additions, using a spatula. Spread the batter evenly onto a baking sheet lined with parchment paper. Bake at 350°F (175°C) for 10 minutes.

When the cake comes out of the oven, turn it out onto a damp kitchen towel, remove the parchment paper, and place the cake onto a new sheet of parchment paper. Brush the cake with the orange syrup.

Take the ganache out of the refrigerator and whip it with a mixer until light and fluffy.

Spread half of the whipped ganache over the cake, then roll it up tightly. Cover the outside of the roll with the remaining ganache and decorate as desired.