Preheat the oven to 375°F/190°C. Put 24 paper liners into shallow muffin pans.
Place the butter, superfine sugar, and vanilla extract in a large bowl and beat together until light and fluffy. Gradually beat in the eggs. Stir in the mashed bananas and sour cream. Sift in the flour, baking powder, and baking soda and, using a metal spoon, fold into the batter with the chopped pecans.
Spoon the batter into the paper liners. Bake in the preheated oven for 20 minutes, or until risen, golden, and firm to the touch. Transfer to a wire rack and let cool.
To make the buttercream, put the butter in a bowl and beat until fluffy. Sift in the confectioners’ sugar and mix together well.
Spoon the buttercream into a large pastry bag fitted with a large star tip. Pipe a swirl of buttercream on top of each cupcake and decorate with a pecan half.
Excerpted with permission from <a href="http://www.amazon.com/gp/product/1445438070/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1445438070&linkCode=as2&tag=craftfocom-20">"The Cupcake: Life's Better With a Cupcake,"</a> published by Parragon Books.