Preheat oven to 350°F. Line the bottoms of two 8-inch round cake pans with parchment paper circles and spray with baking spray or just grease and flour the pans.
In a medium bowl, whisk together flour, baking powder and salt. Set aside.
2 1/4 cups all-purpose flour, 2 1/2 teaspoons baking powder, 1/2 teaspoon salt
In bowl of a stand mixer, cream sugar, butter, and oil together until light and fluffy, about 2-3 minutes, scraping down the sides of the bowl a couple of times during mixing.
1 1/2 cups granulated sugar, 1/2 cup salted butter, 1/2 cup oil
Add vanilla and eggs, one at a time, beating well after each addition.
2 teaspoons vanilla extract, 3 large eggs,
Add the flour mixture and the milk alternately, about 1/3 of each at a time, mixing between additions just until combined.
1 cup whole milk
Remove 1 cup of the batter to a medium bowl. In a separate, microwave-safe bowl, melt the chocolate on short bursts in the microwave, stirring every 20 seconds, until smooth. Pour the chocolate into the yellow cake batter, stirring to combine.
4 ounces bittersweet or semisweet chocolate
Divide the batter evenly between the two prepared round pans, then dollop spoonfuls of the chocolate batter on top, swirling the two batters together with a butter knife.
Bake for 30-34 minutes, until a toothpick inserted into the center of each cake comes out clean.
Remove the cakes from the oven and cool for 10 minutes before running a sharp knife around the edges, then gently turning the cakes out onto a wire cooling rack to cool completely.
Trim the tops of the completely cooled cakes so they are level, then frost with chocolate buttercream and decorate with sprinkles.