Preheat oven to 170°C. Grease a 8-inch cake pan. Set aside.
Sift the flour, baking powder, baking soda and salt together in a large bowl. Set aside.
In stand mixer fitted with a paddle attachment, beat the butter on high speed in a large bowl until smooth and creamy. Add the sugars and vanilla extract, beat on medium-high speed until creamed. Scrape down the sides and up the bottom of the bowl as needed. With the mixer running on low speed, add the dry ingredients in 3 additions, alternating with the milk and mixing each addition just until incorporated. The batter will be slightly thick.
Remove 1/2 cup of vanilla batter and pour into a medium bowl. Add cacao powder, mix until combined.
Spread about half of the vanilla batter in the bottom of the pan. Use an ice cream scoop, scoop dollop of the chocolate batter on top. Alternately add dollop of each of the remaining batters to the cake pan. Using a knife gently swirl the two batters together so they’re mixed but not completely blended.
Bake for 45-50 minutes, or until a toothpick inserted into the center of one cake comes out clean.