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 until uniformly blended.
03 - Beat butter and granulated sugar in a large bowl until pale and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, beating thoroughly 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 mixture. Mix just until combined.
06 - Divide batter evenly among prepared pans and smooth tops. Bake 25–30 minutes until toothpick inserted in center emerges clean. Cool 10 minutes in pans, then turn onto racks to cool completely.
07 - Combine strawberries, sugar, and lemon juice in saucepan over medium heat. Cook 8 minutes until strawberries break down. 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. Incorporate lemon juice, zest, salt, and enough milk to achieve spreadable consistency.
09 - Place first cake layer on serving plate. Spread half the strawberry filling, leaving edges clear. Add second cake layer and spread remaining filling. Position third layer on top.
10 - Frost top and sides with lemon buttercream using offset spatula. Garnish with fresh strawberry slices and lemon zest if desired. Refrigerate at least 30 minutes before slicing for clean layers.