Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans or line them with parchment paper circles. Set aside.
In a medium bowl, whisk together the sifted cake flour, baking powder, and salt. Set aside.
In a large bowl, using an electric mixer, cream the softened butter and granulated sugar together on medium speed for 3-5 minutes, until light, fluffy, and pale in color. Scrape down the sides of the bowl as needed.
Beat in the eggs one at a time, mixing well after each addition. Scrape down the bowl. Add the vanilla bean seeds (or paste) and mix until just combined.
In a separate small bowl or measuring cup, whisk together the buttermilk and whole milk.
With the mixer on low speed, alternately add the dry ingredients and the milk mixture to the butter mixture, starting and ending with the dry ingredients (e.g., add ⅓ dry, mix, add ½ wet, mix, add ⅓ dry, mix, add ½ wet, mix, add ⅓ dry, mix). Mix until just combined after each addition. Be careful not to overmix the batter.
Divide the batter evenly between the prepared cake pans. Smooth the tops with a spatula.
Bake for 28-32 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The tops should be lightly golden and spring back when gently touched.
Allow the cakes to cool in their pans on a wire rack for 10-15 minutes. Then, carefully invert the cakes onto the wire rack to cool completely before frosting.
To make the frosting: In a large bowl, cream the softened butter with an electric mixer until smooth and fluffy. Gradually add the sifted powdered sugar, alternating with milk/cream, until combined and creamy. Beat in the vanilla extract and salt. If frosting is too thick, add a little more milk; if too thin, add more powdered sugar. Beat on high speed for 1-2 minutes until light and fluffy.
Once the cakes are completely cool, level the tops if desired. Place one cake layer on your serving plate or cake stand. Spread about a third of the frosting over the top. Place the second cake layer on top and cover the entire cake with the remaining frosting. Decorate as desired and serve.