01 - Preheat oven to 350°F. Grease and line three 8-inch round cake pans with parchment paper.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
03 - Beat butter and sugar in a large bowl until pale and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, beating well after each addition. Mix in lemon zest, lemon juice, and vanilla extract.
05 - Add dry ingredients in three additions, alternating with milk, beginning and ending with flour. Mix just until combined.
06 - Divide batter evenly among prepared pans. Bake for 25–30 minutes until a toothpick inserted in center comes out clean. Cool in pans 10 minutes, then turn out onto racks to cool completely.
07 - Combine strawberries, sugar, and lemon juice in a saucepan. Cook over medium heat until strawberries break down, about 8 minutes. Stir in cornstarch mixture and cook 1–2 minutes more until thickened. Cool completely.
08 - Beat butter until creamy. Gradually add powdered sugar, beating until smooth. Beat in lemon juice, zest, salt, and enough milk to reach spreadable consistency.
09 - Place one cake layer on a plate. Spread half the strawberry filling, avoiding edges. Top with second cake layer and remaining filling. Place third layer on top.
10 - Frost top and sides with lemon buttercream. Decorate with fresh strawberries and lemon slices if desired. Chill at least 30 minutes before slicing for neat layers.