Preheat the oven to 340°F/170°C (150° fan-assisted) and grease two 8-inch cake pans. I also like to line the bottom of the pans with some parchment paper circles.
Sift the dry ingredients into a bowl, including the flour, baking powder, and salt. Mix until combined.
In a stand mixer, whip the eggs and sugar together on high speed for 5 full minutes until they are thick, pale, and doubled in volume. If you drizzle the batter on top of itself, it should leave a little trail.
Meanwhile, warm the butter, milk, and vanilla either in a saucepan over medium heat or in the microwave in 15-second bursts. It should be just hot enough that the butter melts - do not let it boil. Set aside.
Add half of the flour mixture to the whipped eggs and either fold it in gently with a spatula, or mix it in the mixer on the lowest speed setting. It is crucial that you try not to knock too much air out of the batter, so be very gentle whichever method you choose. Add the reamining flour mixture and repeat.
Gradually pour the hot milk mixture into the cake batter. Either fold it in carefully with a spatula while adding a little at a time, or continue with the mixer on low speed while you pour in a slow, steady stream. Make sure you scrape the bottom of the bowl to ensure everything is well incorporated.
Pour the sponge cake batter into the prepared pans and bake for 30 - 35 minutes or until the top is golden brown and a skewer inserted in the middle of the cake comes out clean.
Let the cake cool in the pan for 10 minutes before inverting onto a wire rack to cool completely.
Notes
You can choose whether to fold in the flour and milk by hand or with a stand mixer. For the best results, I recommend doing this by hand as it is much easier to control and you are less likely to knock the air out of the batter - but both options will work. Just be super careful to be gentle with the batter either way as if the air is knocked out the cake can either deflate or be too dense.