Preheat Oven & Grease Tin: Preheat the oven to 356ºF (180ºC) fan-forced. Grease a 10-cup bundt pan with butter and dust lightly with flour using a tea ball strainer or mini sieve for an even coating.
Mix Dry Ingredients: In a medium-sized mixing bowl, whisk together flour, baking powder, and salt. Set aside.
Beat Sugar, Oil, and Eggs: In a large mixing bowl, use an electric mixer to beat the sugar, oil, and eggs together for 1 minute.
Add Milk and Vanilla: Pour in the milk and vanilla extract. Continue beating for an additional 20 seconds.
Combine with Dry Ingredients: Gradually add the dry ingredients to the wet ingredients. Beat until just incorporated and smooth; be careful not to over-mix. The batter will be relatively thin.
Create Chocolate Batter: Transfer half of the cake batter into another bowl. Sift the cocoa powder over this batter and fold until well mixed with no cocoa lumps remaining.
Layer and Swirl Batter: Pour a layer of vanilla batter into the prepared bundt pan. Alternately add scoops of chocolate and vanilla batter on top, continuing until all batter is used. Gently swirl through the batter with a butter knife or skewer a few times to create a marbled effect.
Bake: Place the pan in the oven and bake for 40-45 minutes, or until a toothpick inserted into the cake comes out clean or with just a few moist crumbs attached.
Cool: Allow the cake to cool in the pan for 15 minutes, then invert onto a wire rack to cool completely.