Preheat the oven to 350°F (175°C). Grease and flour a 9-inch (23 cm) round cake pan.
In a medium bowl, whisk together the sifted all-purpose flour, baking powder, and salt. Set aside.
In a large bowl, beat the softened unsalted butter and granulated sugar together with an electric mixer on medium speed until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Alternately add the dry ingredients and whole milk to the butter mixture, beginning and ending with the dry ingredients. Mix each addition just until combined; do not overmix.
Pour half of the batter into the prepared cake pan and spread evenly.
Spoon the remaining batter over the first layer. Using a knife or skewer, gently swirl the batter to create a marbled effect.
Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan for 10 minutes.
Turn the cake out onto a wire rack to cool completely before slicing and serving.