In a large bowl, whisk together flour, baking powder, and salt.
In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, 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 large liquid measuring cup, whisk together 1 cup 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 1½ cups (330g) of batter in a large 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 ¼ cup 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.