Preheat the oven to 180c. Butter and flour 2 (9-inch) cake pans and line the bottoms with parchment paper. Wrap the pans with cake strips.
In a large bowl, whisk together flour, baking powder, and salt.
In a large mixing bowl of a planetary mixer beat butter on medium speed until creamy. Add the sugar and beat until light and fluffy, about 3 minutes. Add the eggs, one at a time, beating until well combined before adding the next.
In a liquid measuring cup, whisk together 240ml of buttermilk and vanilla.
With the mixer on low, add one-third of the flour mixture. Alternate with half of the buttermilk mixture, and continue alternating with the remaining flour and milk mixture.
Reserve 330g of batter in a bowl. Divide the remaining batter among the two cake pans.
Melt the chocolate in the microwave in 20 second intervals, stirring between each, until smooth. Or melt in a double boiler. Add to the reserved cake batter along with the remaining 20ml buttermilk, and stir until fully combined.
Dollop the chocolate batter over the plain batter in the pans. Using the tip of a knife, swirl both batters together.
Bake for 30 to 35 minutes or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 20 minutes. Remove and cool completely on a wire rack.
