Preheat your oven to 175°C (350°F). Grease a 9x5 inch loaf pan with butter and dust lightly with flour, tapping out the excess.
Using a mixer, cream the softened butter and sugar together on medium speed until the mixture is pale, fluffy, and fragrant, about 3-4 minutes. You’ll notice it turns a lighter color and feels smooth.
Add the eggs one at a time, beating well after each addition. The batter should become silky and increase in volume with each egg. Scrape down the sides of the bowl as needed.
Stir in the vanilla extract, mixing just until combined. The aroma of vanilla will fill your kitchen, and the batter will look smooth and glossy.
In a separate bowl, sift together the flour, cocoa powder, and baking powder. Whisk lightly to combine.
Add the dry mixture gradually to the wet batter, mixing on low speed or folding gently with a spatula until just combined. Be careful not to overmix—this keeps the cake tender.
Divide the batter evenly into two bowls. Mix the melted dark chocolate into one bowl, stirring until smooth and glossy.
Drop spoonfuls of plain batter into the prepared loaf pan, then add spoonfuls of chocolate batter on top. Continue until all batter is used, creating a layered effect.
Using a toothpick or skewer, gently swirl through the batter in a figure-eight pattern. Avoid overmixing—aim for a marbled look with distinct streaks of dark and pale batter.
Bake in the preheated oven for 55-65 minutes. Check doneness by inserting a toothpick into the center; it should come out with moist crumbs, not wet batter.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack and cool completely before slicing. This resting period helps the crumb set and makes slicing easier.
Slice and serve the beautiful marbled cake at room temperature, revealing the intricate swirl pattern inside. Enjoy the moist, tender crumb and rich chocolate flavor with every bite.
