Preheat oven to 350°F (175°C). Grease and line two 8-inch round pans. Whisk flour, baking powder, and salt. Beat butter and sugar until fluffy, about 3 minutes. Beat in eggs one at a time, then vanilla and sour cream. Alternate adding flour mixture and milk, mixing just until smooth.

Divide batter between pans. Bake 25-30 minutes until golden and a toothpick comes out with a few moist crumbs. Cool in pans 10 minutes, then turn out to cool completely.

In a saucepan, combine raspberries, sugar, lemon juice, and cornstarch slurry. Bring to a boil, then simmer 5-7 minutes until thick, stirring often. Cool completely, covering surface with plastic wrap.

Beat butter 2-3 minutes until light. Gradually add powdered sugar on low. Mix in vanilla and milk, then beat on high 3 minutes until fluffy and spreadable.

Place one cake layer on a plate. Spread cooled raspberry filling within ½ inch of the edge. Add second layer. Apply a thin crumb coat, chill 20 minutes, then frost with remaining buttercream.

Slice with a serrated knife and serve at room temperature.