Preheat your oven to 170ºC (340ºF). Line three 20 cm (8-inch) round baking pans with parchment paper and grease the sides lightly.

In a large mixing bowl, sift together the dry ingredients: flour, cornstarch, baking powder, baking soda, and salt.

In a separate bowl, combine the granulated sugar with the lemon zest. Rub together until fragrant, then add softened butter and mix until light and creamy.

Incorporate the eggs into the butter mixture and blend well, then gradually mix in half of the dry mixture.

Add sour cream, vegetable oil, lemon juice, and vanilla extract. Mix until fully integrated, then add the remaining dry ingredients.

Divide the batter among the prepared pans and bake for 20-23 minutes or until a toothpick comes out clean.

Allow the cakes to cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

For the buttercream, beat softened butter until fluffy, then gradually add powdered sugar and strawberry powder, alternating with milk.

Assemble the cake by layering the cake layers with strawberry buttercream, chilling for 30 minutes after applying a crumb coat.

After chilling, frost the entire cake with remaining buttercream and decorate with fresh strawberries and lemon slices.

Slice and serve chilled, allowing each slice to come to room temperature for best flavor.