Preheat your oven to 350°F (175°C). Grease and line a 9×5-inch loaf pan with parchment paper, allowing some overhang for easy removal.
In a medium bowl, sift together the all-purpose flour, baking powder, and salt. Set aside.
In a large mixing bowl, cream the unsalted butter and granulated sugar until light and fluffy, about 3-5 minutes, using a stand mixer or handheld mixer. Scrape down the sides of the bowl as needed.
Add the eggs one at a time, beating well after each addition. Incorporate the vanilla extract.
Step 2: Combine Wet and Dry Ingredients
With the mixer on low speed, alternately add the dry flour mixture and milk to the butter mixture, beginning and ending with the flour. Mix until just combined; avoid overmixing to ensure a tender cake crumb.
Step 3: Create the Marble Effect
Divide the batter into two portions. Stir the melted dark chocolate and cocoa powder into one half of the batter until fully incorporated.
Spoon dollops of vanilla and chocolate batters into the prepared loaf pan, alternating to create a pattern. Use a butter knife to swirl the batters together, creating a marbled effect. Be careful not to over-swirl, or you may lose the distinct marble design.
Step 4: Bake the Cake
Bake for 55-65 minutes, or until a toothpick inserted into the center comes out clean. If the top browns too quickly, cover loosely with foil during the last 15 minutes of baking.
Once baked, remove from the oven and let the cake cool in the pan for 15 minutes. Then, transfer it to a wire rack to cool completely.
Step 5: Prepare the Vanilla Bean Glaze
In a small bowl, whisk together the sifted powdered sugar, vanilla bean seeds, and a pinch of salt. Gradually add milk or heavy cream, one tablespoon at a time, until you achieve a pourable consistency.
Drizzle the glaze over the cooled cake, allowing it to run down the sides for a beautiful finish.
Step 6: Add the Festive Toppings
Sprinkle chocolate shavings over the glazed cake, followed by a light dusting of edible gold dust for a luxurious touch.
