Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans, or line with parchment paper circles and grease the sides.

In a large bowl, whisk together the sifted all-purpose flour, granulated sugar, baking powder, and salt. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream together the softened unsalted butter on medium speed until light and fluffy, about 2-3 minutes.

Add the eggs one at a time, mixing well after each addition until fully incorporated. Scrape down the sides of the bowl as needed.

Gradually add the dry ingredients to the wet ingredients in three additions, alternating with the whole milk, beginning and ending with the dry ingredients. Mix on low speed until just combined after each addition, being careful not to overmix.

Stir in the pure vanilla extract until just incorporated. The batter should be smooth and slightly thick.

Divide the batter evenly between the two prepared cake pans.

Bake for 30-35 minutes, or until a wooden skewer inserted into the center of the cakes comes out clean.

Let the cakes cool in the pans for 10-15 minutes before inverting them onto a wire cooling rack to cool completely. Ensure cakes are completely cool before frosting, about 1-2 hours.

In the bowl of a stand mixer fitted with the paddle attachment, beat the softened unsalted butter on medium speed for 5 minutes until very light and fluffy.

Gradually add the sifted powdered sugar, 1 cup at a time, mixing on low speed until combined, then increasing to medium speed. Continue until all the sugar is incorporated and the frosting is thick.

Add the pure vanilla extract and salt. Beat on medium-high speed for another 2-3 minutes.

With the mixer on low speed, slowly add the heavy cream or whole milk, 1 tablespoon at a time, until the desired consistency is reached. You want a smooth, spreadable frosting. If it's too thick, add more liquid; if too thin, add more powdered sugar. Beat on medium-high speed for 1 minute to ensure it's light and airy.

Once the cakes are completely cool, place one cake layer on your serving plate or cake stand.

Spread about 1/2 to 3/4 cup of vanilla buttercream evenly over the top of the first layer.

Carefully place the second cake layer on top.

Use the remaining buttercream to frost the top and sides of the entire cake. You can use an offset spatula or bench scraper for smooth sides.

Decorate as desired (e.g., sprinkles, fresh berries).