Preheat the oven to 180°C. Grease and flour 3 7-inch cake tins; set aside.
In a medium bowl, sift together the flour, baking powder, and bicarbonate of soda. Whisk together, and set aside.
In the bowl of a stand mixer fitted with a paddle attachment - or in a large bowl with an electric whisk - beat the butter on medium until smooth. Add the sugars, turn to medium-high and beat until the mix is light and fluffy, about 3-5 minutes. Scrape down the bowl. Add the vanilla, and scrape down again.
Turn the mixer to medium-low and add the eggs + egg yolk one at a time, scraping down the sides of the bowl between each addition.
Turn the mixture to low and add the flour mixture in three batches, alternating with the buttermilk, beginning and ending with the flour mixture. After the last batch of flour has been added, be sure not to overmix!
Divide the batter evenly between the prepared tins and bake in the preheated oven for 20-25 minutes, or until a cake tester inserted into the centre of the cake comes out clean. Leave to cool in the tins for 15-minutes before being turned out onto a wire rack to cool completely.