Preheat oven to 350°F (180°C) or 320°F (160°C fan). Grease and line two 8-inch cake pans with parchment paper.

In a large bowl, whisk together the flour, baking powder, and salt. Set aside.

Beat eggs on medium-high speed for 30 seconds, then gradually add the sugar over 45 seconds while beating. Continue mixing until the mixture triples in volume and becomes pale and airy (around 7 minutes).

While eggs are whipping, heat milk and butter in a microwave-safe jug for 2 minutes, or until butter melts. Add vanilla and oil to the hot mixture.

Gently fold one-third of the flour mixture into the egg mixture. Repeat twice more until just combined.

Mix about 1½ cups of the egg mixture into the milk-butter mixture. Whisk until smooth and foamy.

Slowly pour this back into the main egg batter while mixing on low speed. Stir until combined.

Divide the batter between the pans. Tap each pan gently on the counter to release air bubbles.

Bake for 30 minutes or until a toothpick inserted comes out clean. Let cakes cool in pans for 15 minutes, then turn out onto racks.

To make the buttercream, beat butter until light and fluffy. Gradually add powdered sugar, vanilla, and milk until desired texture is reached.

Frost once cakes are completely cool. Level layers if needed for a neater finish.