Preheat oven to 180 degrees C. Grease and flour a 20x20cm cake pan.
In a large bowl, cream butter and sugar until light. Add eggs and beat well. Dissolve bicarb soda in the sour cream and add it to the butter mixture. Beat well. Add the mashed bananas and mix in. Add flour and mix well. Stir in vanilla essence.
Pour batter into pan and bake in preheated oven for 45 minutes or until done.