Preheat the Oven:
Preheat your oven to 180°C (or 160°C for fan ovens). Grease three 20 cm springform pans with butter and dust with flour.
Prepare the Lemons:
Wash the lemons with hot water, dry them, and zest them. Squeeze the juice from one lemon.
Mix the Butter and Sugar:
In a mixing bowl, cream the soft butter with sugar until light and fluffy. Beat in the eggs one at a time.
Combine the Flavors:
Add the lemon zest, juice, poppy seeds, and vanilla extract into the butter-sugar mixture, stirring well.
Incorporate the Dry Ingredients:
Sift the flour, cornstarch, baking powder, and baking soda in a bowl. Gradually mix this into the batter in three portions until smooth.
Bake the Cakes:
Divide the batter evenly among the prepared pans and bake in the preheated oven for 25-30 minutes or until golden brown. Allow them to cool completely.
Prepare the Frosting:
For the frosting, beat the soft butter, add powdered sugar gradually, and mix until light and fluffy. Stir in the cream cheese and zest from the remaining lemon.
Assemble the Cake:
Level the cooled cakes. Spread frosting between each layer and over the top. Garnish with lemon slices and poppy seeds.