Pre-heat oven to 190 deg C (375 deg F). Place oven rack in the centre of the oven. Lightly grease the base and sides of an oval loaf pan (9 in x 4 in x 2 1/4 in).
Sift together the flour, baking powder, and salt. In a small bowl, combine whole eggs and egg yolks, and stir with a fork to mix well. In another bowl, combine the milk, cream, and vanilla extract. Lastly, in a separate bowl, combine the cocoa paste ingredients and stir until smooth.
Using an electric mixer fitted with a paddle attachment, cream butter and sugar on medium speed (speed 3 on my Kitchen Aid mixer) until light and fluffy, about 10 minutes.
Keeping the mixer speed on low to medium, add the eggs, little by little, beating well after each addition. Add the flour mixture, alternating with the milk-cream mixture, in 1/4 portions each time, beating well until you get a smooth batter. Do not over-beat during this stage.
Weigh out 100 g of the cake batter. Gently fold in the cocoa paste until well combined. In the prepared loaf pan, pour enough plain batter to cover the base. Next, alternate dollops of chocolate batter with plain batter, ending with some chocolate batter on the top. Using a toothpick or bamboo skewer, create a swirled or marbled effect.
Bake in the oven for 30 to 35 minutes, or until a metal or bamboo skewer inserted into the centre of the cake, emerges free of batter. When cake is done, remove from oven, and let cool in the pan for 5 minutes. Remove cake from the pan, and cool completely on a cooling rack. Slice, and serve as desired.