Preheat oven to 350°F. Line 2 (9-inch) cake pans with parchment; grease paper.
Cream 1/2 pound butter and 2 cups sugar. Beat in 4 eggs, 2 tsp. vanilla, 1 Tbsp. baking powder, 1 tsp. salt and 1/2 tsp. baking soda. Add 3 cups flour and 2 cups buttermilk alternately. Fold in 1/2 pound chopped strawberries. Scrape into pans.
Bake until a tester comes out clean, 45 to 50 minutes.
Make buttercream while cakes cool. Bring 1 pint sliced berries and 1/3 cup water to a boil, stirring; remove from heat. Beat 4 yolks and 1 1/2 cups confectioners' sugar; stir into liquid. Stir in 1/2 tsp. salt; cook, whisking, until temperature reaches 170°F. Off heat, beat in 1/2 Tbsp. lemon juice; let cool. Beat in 1 pound butter 2 Tbsp. at a time until fluffy. Add a drop of red coloring, if desired.
Place one cake layer on a platter. Spread 3/4 cup buttercream on top. Add top layer and frost cake.