Heat your oven to 160°C Fan/180°C Gas/ 355°F/Gas mark 4.
Grease your tin and line the base with a piece of greaseproof paper.
Sift together your flour, baking powder and salt and set aside.
If using a hand mixer fix the beaters and if using a stand mixer fit the paddle attachment. Beat your butter a little, just until it spreads around the outside of the bowl and no longer clumps around the beaters, so that you know it is nice a soft.
Add your sugar and beat on a slow setting for a minute, turn up to a medium speed for a minute and then beat on a fast setting for a minute, stop as soon as the mixture has turned a pale shade of yellow, it is very soft and easily spreadable. Intermittently keep wiping down the sides and base of the bowl with your spatula, to ensure everything is equally mixed.
Add a egg (must be at room temperature and not straight from the fridge) and a dessert spoonful of your sifted flour mixture and beat on a low setting until the egg has been fully mixed in. Again, keep wiping down the bowl to be sure the mixture is all equally incorporated.
Repeat with each remaining eggs and a little flour mixture each time.
Mix together your room temperature sour cream, milk and vanilla extract and add it all to your cake batter. Start beating in on a low setting and then turn up to medium, just until incorporated. Don't worry if the mixture looks a little curdled, it is meant to be that way.
Add all of your flour mixture and beat in on a low setting, stopping to scrape the bowl down.
As soon as the flour has disappeared turn the mixer up and beat on fast for no more than 10 seconds, by which time the cake batter should be smooth and silky. don't be tempted to beat any longer, otherwise, the gluten in the flour will develop and your cake will be rubbery.
Pour your vanilla cake batter into your tin and spread it out evenly.
Place in the centre of your oven and bake for 30 minutes or until a skewer inserted into the centre comes out clean. Remove and place in the cake still in its tin on a cooling rack.
If the cake has risen in the centre use an implement such as a fish slice to gently push it back down, you will need to do this whilst it is still hot and pliable.
Run a knife around the edge of the tin to ensure the cake does not stick. After 5-10 minutes place a cooling rack over the tin and turn it over so that the cake flips onto it. Then place another cooking rack onto the cake itself and flip the cake over again so that the right side is facing up.
Leave to fully cool.