Preheat the oven to 180 °C (350 °F). Separate the eggs. Beat the egg whites until stiff and beat the egg yolks with sugar and orange zest until pale. Gradually add orange juice and oil and mix until smooth. Stir the flour with the baking powder and sift to the egg yolks. Stir with a mixer to combine. Stir in the egg whites with a spatula. Spread the batter evenly on an 18 x 28 cm baking tin lined with parchment paper (the bottom). Bake for 20 minutes. Take the baked sponge cake out of the baking tray and cool it on a cooling rack.
Peel the oranges, cut them into small pieces, remove the seeds and blend them in a blender. Cut the cool sponge lengthwise into two equal layers. Place the bottom layer of the sponge cake on a stand and fit the ring. Crush the top layer in a larger bowl and add the blended oranges, orange juice, sugar, jam and cacao. Mix well and spread the mixture evenly over the sponge cake. Level out the surface until smooth and pour over the chocolate glaze. For the glaze, melt the chocolate and oil in a double boiler.
Place the cake in the fridge to allow the chocolate to set. Cut the cooled cake into squares of desired size.