Preheat oven to 180 degrees C. Prepare 3 x 8″ (20cm) cake rounds with cooking spray and line the base with parchment.
Sift cake flour, baking powder, baking soda, salt and poppy seeds and set aside.
Cream butter, oil, sugar, vanilla and lemon zest together until the mixture becomes very pale and fluffy.
Beat in the eggs one at a time, mixing well after each addition.
Mix buttermilk and lemon juice and set aside.
Add half of the flour mixture to the butter mixture and beat until well combined. While the mixer is running at a medium speed, add all of the buttermilk mixture. Beat in the rest of the flour mixture. Mix until all ingredients are well combined.
Divide batter among the three prepared cake pans and spread into an even layer. Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, about 20 – 25 minutes.
Remove from the oven and let cool in the pans for 5 minutes. Invert cakes onto wire racks and allow to cool down completely before frosting.
Lemon Crème Pàtissière
In a small measuring cup, mix the maizena, cake flour and about 60ml of the milk. Mix until lump free and set aside.
In a microwaveable mixing bowl, beat the egg and the rest of the milk until well combined.
Add the lemon juice, vanilla and the maizena mixture and mix well.
Next, add sugar and salt, mix until well combined.
Microwave on high for 4 – 5 minutes. Stop every minute and whisk the mixture.
When the creme pat is done, add the butter and whisk until butter is evenly combined.
Cover with plastic wrap and set aside to cool to room temperature, and then refrigerate.
Place softened butter into the bowl of a stand mixer that has been fitted with the paddle attachment.
Turn the mixer on a medium setting and cream the butter until it is smooth and has lightened in color, about 3 minutes.
Add icing sugar, half a cup at a time. After each cup has been incorporated, turn the mixer onto the highest speed setting and for about 30 seconds to lighten the frosting.
Add lemon juice and a pinch of salt and mix until well-incorporated.