Preheat oven to 350°F (175°C). Butter and line two 8-inch round cake pans. Melt 4 oz white chocolate gently, then set aside to cool to room temperature.

Whisk together flour, baking powder, and salt in a bowl.

Using a stand mixer, cream soft butter and sugar on medium-high until pale and fluffy, about 3 minutes. Add eggs one at a time, then mix in oil and vanilla on low.

On the lowest speed, alternate adding the flour mixture and buttermilk in three parts, beginning and ending with flour. Fold in the cooled white chocolate by hand until no streaks remain.

Divide batter between pans and bake 25 to 28 minutes, until tops spring back and a toothpick has a few moist crumbs. Cool 10 minutes in pans, then remove and cool completely on a rack.

Melt 6 oz white chocolate and cool. Beat softened butter until creamy, add powdered sugar and salt, whip 3 minutes, then beat in cooled white chocolate until light and spreadable.

Place one cake layer on a plate, spread with frosting, top with second layer. Apply a crumb coat, chill 20 minutes, then finish frosting. Slice and serve at room temperature.