Preheat oven to 140°C. Position a rack on the second lowest shelf of the oven. Brush a round 25cm (base measurement) cake pan with the melted butter to lightly grease. Line base and sides with non-stick baking paper.
Place the self-raising flour, plain flour, sugar, butter, milk, eggs and vanilla essence in a large mixing bowl. Use an electric beater to beat on low speed for 30 seconds or until just combined. Increase the speed to high and beat for 1-2 minutes or until the mixture is thick and all the butter is incorporated. Spoon the mixture into the prepared pan and smooth the surface with the back of a spoon.
Bake in preheated oven for 2 1/2 hours or until a skewer inserted into the centre of the cake comes out clean. Remove from oven and set aside for 10 minutes. Turn onto a wire rack for 2 hours or until completely cool.
Notes & tips
Have the butter at room temperature, soft enough to beat, but not melted.
The undecorated cakes will keep in an airtight container for up to 3 days.