Preheat oven to 180C.

Grease a 7cm deep, 19cm square cake pan and line with baking paper.

Beat softened butter, caster sugar, and vanilla extract using an electric mixer until pale and creamy.

Add eggs, one at a time, beating well after each addition.

Gently fold in sour cream and mashed bananas with a metal spoon.

Sift flour and bicarbonate of soda over the batter and gently fold through.

Spoon batter into the prepared cake pan and smooth the surface.

Bake for 50 to 60 minutes, or until a skewer inserted into the center comes out clean.

Let the cake stand for 5 minutes before turning it out onto a wire rack to cool.

To make the icing, place chopped white chocolate and sour cream in a heatproof bowl over a saucepan of simmering water.

Heat, stirring constantly, for 3 minutes or until the chocolate is smooth and melted.

Remove from heat and let stand for 10 minutes, or until slightly thickened.

Pour icing over the cooled cake, allowing it to drizzle down the sides.

Serve warm.