Preheat oven to 180°C. Brush a round 20cm (base measurement) cake pan with melted butter to grease. Line the base and side of the pan with non-stick baking paper.
Place the butter, sugar, banana, eggs and vanilla in the bowl of a food processor, and process for 2 minutes or until well combined. Add the buttermilk and process until combined.
Add the flour and bicarbonate of soda, and process until just combined. Pour the mixture into the prepared pan. Shake the pan to settle the mixture.
Bake in oven for 1 hour or until a skewer inserted into the centre comes out clean. Set aside in the pan for 10 minutes to cool slightly, before transferring the cake to a wire rack to cool completely.
Frost the cooled cake with Philadelphia Lemon Cream Cheese Frosting (or prepare the original frosting recipe used with this cake). Peel and thinly slice the 1 extra banana and brush 1 cut side of each slice with lemon juice. Arrange the banana slices, lemon juice side up, around the edge of the frosting to serve. Serve immediately. Store leftovers in the refrigerator 2-3 days.