1. Combine chopped strawberries, sugar, and lemon juice in a medium saucepan over medium heat. Cook for 8 to 10 minutes, stirring often, until berries break down and release juices. Stir in cornstarch slurry and simmer 2 to 3 more minutes until thick like jam. Remove from heat, cool completely at room temperature for 1 hour or chill 30 minutes.

3. Preheat oven to 350 degrees F (175 degrees C). Grease three 8-inch round cake pans, line bottoms with parchment, grease again, and dust with flour. Whisk flour, baking powder, baking soda, and salt in a large bowl. In a stand mixer with paddle attachment, beat butter, oil, and sugar on medium speed until light and fluffy, about 2 minutes. Add eggs one at a time, beating well after each. Mix in vanilla. On low speed, add dry ingredients in three parts alternating with buttermilk, starting and ending with dry. Scrape bowl, mix just until combined, no more than 30 seconds. Divide batter evenly into pans, about 2 cups each. Tap pans on counter to release air bubbles. Bake 24 to 30 minutes until toothpick in center comes out clean. Cool in pans 10 minutes, then turn onto wire racks to cool fully, about 2 hours. Level tops if domed.

5. Beat softened butter on medium speed until creamy, 2 minutes. Add powdered sugar 1 cup at a time on low, scraping sides as needed. Once thick, beat in 1/4 cup cooled strawberry filling, heavy cream, vanilla, and salt. Taste and add more sugar if needed for sweetness. Whip on high 2 minutes for light texture. If too soft, chill 15 minutes.

7. Place one cake layer on serving plate. Pipe a buttercream border around edge to dam filling. Spread 1/2 cup strawberry filling inside border. Top with second layer, repeat filling. Add third layer. Apply thin crumb coat of buttercream over top and sides. Chill 20 minutes. Frost generously with remaining buttercream, reserving some for decoration. Pipe swirls on top, add halved strawberries if desired. Chill 30 minutes before slicing for clean cuts.