Beat the butter and sugar together in a bowl, with a hand held electric mixer, until creamy. Then beat in the egg till well incorporated. Fold in the mashed banana, then sift the flour, baking powder and salt into the mixture. Fold again till everything is well mixed.
Spoon the batter about halfway into 8 paper liners. Place one slice of banana in each liner and top with remaining batter such that the banana pieces are completely covered.
Bake at 180C for about 16 to 20 minutes until risen and a skewer inserted in the center comes out clean. Cool the cupcakes on a wire rack.
While the cupcakes are cooling, prepare the toffee/ lightly caramelized condensed milk. To do this, melt the butter in a saucepan, over low heat. Add the salt and condensed milk and cook, stirring continuously, till it turns a light caramel colour and thickens a bit. Allow to cool.
Now assemble the cupcakes
I used the cone method to fill these cupcakes. While removing a bit off the cones ensure that you don't remove the banana slice which was cooked in the cupcakes.
Fill the cupcake with the caramelized condensed milk. Cover the filling with the piece of cupcake which was cut out, and press slightly to seat it well.
Cover the cupcakes with whipped cream and decorate with grated chocolate. It would be a good idea to refrigerate these cupcakes till they're ready to be served.
As you can see form my pictures, it was too warm and the cream started flowing a bit. So the cupcakes don't look as pretty as they should, but they taste just great.