Pour 2-3 cm of water into a saucepan and bring to a boil. Add the sweet potato to a steaming basket, then place the steamer in the pan, making sure that the water doesn’t touch the bottom of the steamer. Cover and steam for 10 minutes or until the sweet potato is completely soft. Remove the sweet potato from the steamer and transfer to a bowl. Mash until completely puréed.

Preheat the oven to 340 °F.

Place the flour, baking powder and salt in a bowl and whisk together to combine well.

In another bowl, combine the sugar and melted butter and whisk until blended. Whisk in the eggs, one at a time, followed by the vanilla. Fold in the flour mixture, a third at a time, stirring well after each addition.

Divide the mixture in half, separating it into two bowls. To one bowl of batter, add the mashed sweet potato and fold until combined. To the other bowl of batter, add the black sesame paste and milk and fold until combined.

Prepare your bundt pan right before you are ready to fill it. Grease a 10–12 cup capacity bundt pan with non-stick oil spray, making sure to pay extra attention to any details and crevices in your pan, then scatter with sugar. Shake the pan to evenly distribute the sugar, then remove any excess sugar by tipping the pan upside-down over the sink.

Add spoonfuls of the two batters alternately to the bundt tin, distributing it evenly. Run a single chopstick or knife through the mixture to create a marbled effect.

Bake in the middle of the oven for 1–1 ¼ hours. Start checking the cake for doneness at the 1-hour mark. Insert a toothpick or bamboo skewer into the cake and if it comes out clean, it is ready.

Leave the cake in the pan on a wire rack to cool for 10 minutes, before turning out to cool completely. Once cooled, dust the cake with a little icing sugar.