Line a 12-section bun tray with paper cake cases. Put the butter, caster sugar, eggs, flour and baking powder in a bowl and beat with a hand-held electric whisk for about a minute until light and creamy. Stir in the mashed banana and sultanas. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and just firm. Transfer to a wire rack to cool.
Line a bowl with a double thickness of kitchen paper. Spoon the yogurt on to the paper. Bring up the edges and gently squeeze out as much liquid as possible. Tip the thickened ball of yogurt on to 2 more sheets of kitchen paper and squeeze out a little more liquid if possible.
Sift the icing sugar into a separate bowl and add the thick yogurt. Mix well to make an icing with a slightly fudgy texture. Swirl the mixture over the cakes and scatter with sugar sprinkles to decorate.