Preheat the oven to 180°C (350°F) and line the cake pan with parchment paper.

Whisk the all-purpose flour, corn starch, poppy seeds, baking powder, and salt in a mixing bowl.

Rub the granulated sugar with half of the lemon zest using your fingers until fragrant.

Add 3 eggs to the lemon sugar and mix briefly until combined.

Pour in the milk, vegetable oil, and 100 ml lemon juice, then mix until smooth.

Fold the dry mixture into the wet mixture until no dry flour remains.

Pour the batter into the lined cake pan and level the surface.

Bake the cake for 20–25 minutes until set and lightly golden.

Remove the cake from the oven and cool it completely in the pan.

Whisk 2 eggs, 80–100 g granulated sugar, salt, the remaining lemon juice, and remaining lemon zest in a saucepan.

Cook the lemon mixture over low heat while stirring constantly until thickened.

Remove the lemon curd from the heat and stir in the butter and vanilla extract until melted.

Cover the lemon curd directly on its surface with plastic wrap and cool completely.

Whip the whipping cream until softly thickened but not fully stiff.

Fold the sour cream and powdered sugar into the whipped cream until smooth.

Spread the cream over the cooled cake in an even layer.

Spoon the cooled lemon curd over the cream and chill before slicing.

Store leftover cake covered in the refrigerator.