In a bowl, sieve the dry ingredients (flour, baking powder and baking soda) and set them aside to be used later.
In another bowl, whisk butter until light and fluffy, add in the sugar and whisk until combined. This should take about 4-5 minutes with an electric mixer and longer with hand. Do not skip this step or cut corners. Read why it's important here.
Once the sugar and butter are mixed, add in the vanilla extract and mix it in.
Preheat your oven at 180 degrees Celsius for 10 minutes.
Add the dry ingredients to the butter mix little by little. Keep alternating with buttermilk. When you run out of buttermilk start alternating with milk until you form a nice batter. Do not over mix the batter. Use only a spatula and not a mixer for this step. It's important to only mix until you no longer see dry flour. Once the flour is incorporated, stop or you'll knock the air out.
Pour the batter in a 9-inch greased round tin. (Learn how to successfully grease your tins here)
Bake at 180 degrees for 30-35 minutes or until a toothpick inserted in the center of the cake comes out clean with a few crumbs.
Let the cake cool down slightly before taking it out of the cake tin.