1. Make the Vanilla Cake:

1. Preheat oven to 350°F (175°C). Grease and line two 9-inch cake pans with parchment paper.
2. In a bowl, whisk together flour, baking powder, and salt. Set aside.
3. In a large mixing bowl, beat butter and sugar together until light and fluffy (about 2-3 minutes).
4. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
5. Gradually add the dry ingredients, alternating with milk and sour cream, beginning and ending with the flour mixture. Mix until just combined.
6. Divide the batter evenly between the two cake pans.
7. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.
8. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.

2. Prepare the Strawberry Filling:

1. In a saucepan, combine chopped strawberries, sugar, and lemon juice over medium heat.
2. Cook for 5 minutes, stirring frequently, until the strawberries release their juices.
3. Add the cornstarch mixture, stirring until the sauce thickens. Remove from heat and let it cool.

3. Make the Vanilla Buttercream Frosting:

1. In a mixing bowl, beat butter until smooth and creamy.
2. Gradually add powdered sugar, one cup at a time, mixing well.
3. Stir in vanilla extract and salt.
4. Add heavy cream, one tablespoon at a time, until desired consistency is reached.

4. Assemble the Cake:

1. Place one cake layer on a serving plate. Spread a thin layer of buttercream frosting on top.
2. Spoon the strawberry filling over the frosting, spreading evenly.
3. Place the second cake layer on top. Frost the top and sides with the remaining buttercream.
4. Garnish with fresh strawberries and drizzle with strawberry sauce if desired.