Grease your cake pans with baking spray or butter. I placed parchment papers in them for easier release.
Add oil to the milk, stir then set aside. Add eggs and vanilla to the other portion of milk. Whisk well to combine.
Measure flour and sugar, then place them into the bowl of your mixer. Add baking powder, baking soda and salt. Mix at low speed 1.
Gradually add the chunks of butter. Mix until the batter looks like coarse sand.
Add milk + oil mixture then adjust speed to medium 2. Whip for 2 minutes. Scrape down the bowl as you go along to ensure even distribution of dry and wet ingredients.
Turn speed to low. Slowly pour in the milk + egg mixture. Mix until just combined until the batter is thick. It shouldn’t be runny.
Preheat oven to 168C. Divide batter into cake pans, filling each pan 3/4 full. Gently shake and tap the pan on a table to burst air bubbles and even out the surface of the batter. Bake for 30 minutes until a toothpick inserted comes out clean.
Remove cakes from the oven. Tap them again to release air and prevent shrinking. Let cool on a cooling rack.
Remove cake layers from parchment paper and wrap them in plastic wrap then place them in the fridge for 30-35 minutes to firm up. If you're not frosting them immediately, you can keep them overnight in the fridge or freezer. Just make sure to take your cake out of the fridge a few hours before you plan on frosting it. Make your frosting by combining egg whites and icing sugar on low speed. Adjust to high and whip for 5 minutes. Add vanilla and salt.
Add butter and whip until batter is curdled. Keep whipping on high for 8-10 minutes until it turns white, light and shiny. Turn to low speed and mix for another 20 minutes for smooth, creamier buttercream.
Divide the frosting to 3 large bowls. Add colouring, adjusting the intensity as you like. I opted for 3 colours - white, and two shades of pastel pink.
Remove chilled cakes from fridge. Shave away the brown bits and trim the dome off with a serrated knife until they stack nicely and level. Fill them with frosting in between layers and frost the outside of the cake.
With the remaining frosting, decorate the outside of the cake with buttercream flowers. I used a palette knife to create these floral shapes with different shades of pink.