Preheat oven to 350°F (175°C). Grease and line three 8-inch round cake pans; lightly flour the sides.

Whisk cake flour, baking powder, and salt in a bowl; set aside.

In a stand mixer fitted with a paddle, beat butter and sugar on medium-high until pale and fluffy, about 4–5 minutes.

Add eggs one at a time, scraping the bowl between additions. Mix in vanilla.

With mixer on low, add dry mix in three additions alternating with milk (start and end with dry). Fold in sour cream if using. Do not overmix.

Divide batter evenly between pans. Smooth tops and bake 22–28 minutes, or until a toothpick comes out with a few moist crumbs. Rotate pans halfway through for even browning.

Let layers cool in pans 10 minutes, then invert onto wire racks to cool completely.

In a heatproof bowl, whisk egg whites and sugar together. Set over a simmering pot of water (double boiler) and whisk until sugar dissolves and mixture reaches 160°F.

Transfer to a stand mixer and whip on high until glossy stiff peaks and cooled to room temperature (~8–10 minutes).

With mixer on low, add butter pieces gradually. Increase speed to medium-high until smooth and silky. Mix in vanilla and salt.

Level cake layers with a serrated knife or cake leveler.

Place first layer on a cake board, brush lightly with simple syrup, spread 1/3 cup jam, add a thin layer of buttercream to seal.

Repeat with remaining layers. Apply a thin crumb coat of buttercream and chill for 15–30 minutes to set.

Apply a final smooth coat and use a piping bag for decoration. Chill briefly to set decorations.