Stir together 180ml of milk and 1 tablespoon of lemon juice. Set this aside for 5 to 10 minutes to let it curdle slightly.

In a medium bowl, combine 270g of cake flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ½ teaspoon of salt. Stir briefly, then set aside.

In a large mixing bowl, beat 150g of butter, 30ml of oil, and 130g of sugar on high speed until the mixture is pale and fluffy, about 5 to 6 minutes.

Add the eggs one at a time, beating well after each addition. Then, add 1 tablespoon of vanilla extract and whisk again until everything is well blended.

Sift the flour mixture from Step 2 into the wet mixture in three additions, alternating with the milk from Step 1 in two additions (flour-milk-flour-milk-flour). Mix gently after each addition until just combined. For the final flour scoop, mix at low speed for about 30 seconds.

Evenly divide the batter among three prepared 16cm (6-inch) pans. Bake at 338°F (170°C) for 30 to 35 minutes. Poke a toothpick into each cake's center to see if it comes out crumb-free.

Let the cakes cool in their pans for 10 minutes, then turn them out onto a wire rack to cool completely.

In a heatproof bowl, add 3 egg whites and place the bowl over a pot of simmering water. Add 140g of powdered sugar and a pinch of salt, then whisk constantly until the sugar dissolves and the mixture reaches 60°C (140°F).

Remove the bowl from the pot, then whip the mixture at medium speed until stiff peaks form and it cools to room temperature (about 10 minutes).

Gradually add 300g of room-temperature butter, a few pieces at a time, while beating on medium speed.

Add 1 tablespoon of vanilla extract and a small amount of purple food coloring. Beat after each addition until fully blended.

In a small saucepan, combine 100ml of water and 30g of sugar. Heat gently until the sugar completely dissolves, then let it cool to room temperature.

Lightly brush each cake layer with the sugar syrup.

Place the first layer of cake on a serving plate. Spread a layer of Swiss buttercream on top and smooth it out with an offset spatula. Repeat with the second layer.

Place the final cake layer on top, then spread the remaining buttercream over the top and sides of the cake. Smooth it out with a flexible spatula or knife.

Decorate the cake however you like. Slice and enjoy!