Preheat the oven to 350°F. Grease two 8-inch round cake pans and line the bottoms with parchment paper.
Whisk together the flour, baking powder, and salt in a medium bowl.
Beat the softened butter and sugar until pale and fluffy, about 3 to 4 minutes.
Add the eggs one at a time, mixing after each addition. Mix in the vanilla.
Add the dry ingredients alternately with the milk, beginning and ending with the dry ingredients. Mix only until combined.
Divide the batter evenly between the prepared pans.
Bake for 25 to 30 minutes, or until the tops spring back lightly and a toothpick inserted into the centers comes out clean or with a few moist crumbs.
Cool the cakes in their pans for about 10 minutes, then transfer them to a cooling rack. Let them cool completely
If necessary, trim the domed tops with a serrated knife to create flat layers.
For the frosting, beat the butter until creamy. Gradually add the powdered sugar, then mix in the vanilla, salt, and enough cream or milk to reach a smooth spreading consistency.
Place one cake layer on a cake board or serving plate. Spread an even layer of buttercream over the top. Add fruit jam if desired.
Place the second cake layer on top and gently press it into position.
Apply a thin crumb coat around the top and sides. Refrigerate for 15 to 20 minutes.
Apply the remaining frosting and smooth the top and sides with an offset spatula or cake scraper.
Decorate with fresh berries, chocolate shavings, sprinkles, or additional buttercream. Chill briefly before slicing if desired.