Preheat the oven to 330°f / 165°c. Grease and line a 9" (23cm) square baking pan, extending the parchment paper over the edges of the pan so that you can use them as a sling later to remove the cake. I like to clip down the edges with little binder clips.

In the bowl of a stand mixer or in a large bowl using an electric mixer, cream together the butter, sugar, and vanilla on medium to high speed until very light and fluffy. Add the eggs one at a time, mixing to combine between each, then beat the mix for a further 2-3 minutes on high speed until it has thickened up, and is very light.

Measure out the sour cream into a small bowl. In a medium bowl, combine the flour, baking powder, and salt.

Alternately add the flour mixture and the sour cream, starting and ending with the flour - add the flour in three additions and the sour cream in two, and briefly mix between additions. Switch to a spatula and give the mixture a few turns to ensure that it is evenly mixed.

Transfer the mix to the prepared pan and smooth down with a spatula or the back of a spoon, making sure that there are no large air bubbles in the mixture.

Sprinkle the top with extra granulated sugar if using.

Bake the cake for 45-50 minutes, until the middle springs back when lightly pressed with a finger, and a skewer inserted into the middle of the cake comes out clean or with a few moist crumbs attached.

Remove the cake from the oven and leave to cool in the pan for 15-20 minutes, then use the parchment paper sling to remove it from the pan and allow it to cool completely on a wire rack.

Store the cake at room temperature in an airtight container for up to 4 days.