Preheat the oven to 170c. Grease and flour two 20cm/8" cake tins and line the bottom of each tin with a round of baking paper. Whisk the flour and baking powder together and set aside.
Add the sugar to your mixer (blending fine if necessary). Add the room temperature butter and whip or blend until fluffy and light (pictured).
With the machine running, add the eggs one at a time, making sure each egg has been absorbed before adding the next. If you're using a food processor here, be careful not to over blend. Food processors are strong, so only run the machine as long as needed to blend in the eggs. When all the eggs are absorbed, the mixture will look shiny and creamy (also pictured).
Scrape the egg mixture into a large bowl and add half of the flour mixture. Carefully combine the two, taking cake not to over mix. A whisk gently stroked through the mixture until just blended is good for this, or use a spatula. Repeat with the rest of the flour mixture.
Add the vanilla extract and drizzle in about 2 tablespoons of milk to loosen the batter. When it's ready, a blob of batter should fall of the whisk or spatula when flicked.
Divide the batter between the two cake tins and gently smooth the tops. Bake for about 20 minutes, without opening the oven until you start to check if they are ready. They are done when they are risen and golden, the cake springs back when pressed and a toothpick poked into the sponge comes out clean. Rest the cakes in their tins for about 5 minutes, then turn out onto a cooling rack. Leave to cool completely.