Preheat oven to 180 C / Gas 4. Grease a 20cm square cake tin and line base and sides with baking paper.
Beat butter and sugar in a small bowl with electric mixer until light and fluffy. Beat in eggs, one at a time. Transfer to a large bowl. Stir in sifted flour, banana and buttermilk until combined.
Spread half the cake mixture into prepared tin. Dollop teaspoons of the caramel, about 2cm apart, all over cake mixture. Top with remaining cake mixture and gently spread to level out the surface.
Bake cake in preheated oven for about 50 minutes or until a skewer inserted into the centre of the cake comes out clean. Stand cake in tin for 5m minutes; turn out of tin and set on a wire rack, top side up, to cool a little more before serving.