1. Preheat the oven to 350°F (177°C). Grease and line three 8- or 9-inch round cake pans with parchment paper.
2. Whisk together all-purpose flour, cornstarch, baking powder, baking soda, and salt; set aside.
3. In a large bowl, beat softened butter and sugar until creamy and light, about 2-4 minutes.
4. Add eggs one at a time, mixing well, then add vanilla, lemon zest, and juice.
5. On low speed, mix in dry ingredients alternately with milk, starting and ending with flour. Do not overmix.
6. Divide batter among pans, baking for 21-33 minutes or until a toothpick comes out clean.
7. Cool cakes in pans for 15-20 minutes, then transfer to wire racks to cool fully. Frost when completely cool.

Frosting Preparation:
1. Beat butter until creamy, add cream cheese, and blend until smooth.
2. Gradually add confectioners’ sugar, lemon juice, vanilla, and salt. Beat on high for 3 minutes until fluffy.
3. Frost cake between layers and around the sides evenly.

Assembly:
1. Level cakes if needed. Place one layer on a serving plate, spread frosting, and stack the next layer.
2. Frost the sides and top. Garnish if desired.
3. Refrigerate the cake for 30-45 minutes before slicing.
4. Store leftovers covered in the fridge for up to 5 days.