Preheat oven to 180°C/350°F. Place shelf in the middle of the oven.

Whisk flour, baking powder, and salt in a large bowl. Set aside.

Beat eggs for 30 seconds on speed 6 of a stand mixer fitted with a whisk attachment, or hand beater.

With the beater still going, pour the sugar in over 45 seconds.

Then beat for 7 minutes on high speed, until tripled in volume and white.

While the egg is beating, place butter and milk in a heatproof jug and microwave to melt the butter. Do not let the milk bubble and boil. Do not let the milk cool.

When the egg is whipped, scatter ⅓ of the flour across the surface, then beat on Speed 1 for 5 seconds. Add half the remaining flour, then mix on Speed 1 for 5 seconds. Add the remaining flour, then mix on Speed 1 for 5–10 seconds until the flour is just mixed in. Once you can't see flour, stop straight away.

Pour the hot milk, vanilla, and oil into the now-empty flour bowl. Add about 1½ cups (2 ladles or so) of the egg batter into the milk-butter mixture (doesn't need to be 100% accurate). Use a whisk to mix until smooth — you can be vigorous here. It will look foamy.

Turn the beater back on Speed 1, then pour the milk mixture into the egg batter over 15 seconds, then turn the beater off.

Scrape down the sides and base of the bowl. Beat on Speed 1 for 10 seconds — the batter should now be smooth and pourable.

Pour the batter into the pan.

Bake for 30 minutes.

Remove from the oven and cool for 15 minutes.

While the sponge cake is cooling, prepare the buttercream icing.

Beat the butter with a paddle attachment in a stand mixer for 3–4 minutes on high, until it becomes white and fluffy.

Add the icing sugar gradually, beating slowly, then once mostly incorporated, beat on high for 3 minutes until fluffy.

Add the wild blueberry juice, vanilla, and milk, then beat for another 30 seconds.

Decorate your sponge cake!