Preheat your oven to 350°F (175°C). Thoroughly grease your bundt pan with butter or baking spray and dust with cocoa powder.
In a large bowl, beat the eggs, sugar and vanilla extract until light and fluffy, about 3-4 minutes.
In a separate bowl, whisk together the flour, baking powder, ground walnuts, and cocoa powder.
Melt the dark chocolate using a double boiler or in short bursts in the microwave, stirring frequently. Let it cool slightly, then add to the wet ingredients.
Add the melted chocolate and marmalade to the egg mixture and mix well. Gradually fold the dry ingredients into the wet ingredients, alternating with the orange juice and oil.
Pour the batter into the prepared bundt pan.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Allow the cake to cool in the pan for EXACTLY 10 minutes. After 10 minutes, place a cooling rack on top of the bundt pan and flip it over in one confident motion.
Allow the cake to cool completely, then decorate as desired and serve.
