Preheat the oven to 180C/170C Fan/Gas 3 and line two 12-hole cupcake tins with paper cases.
Put the butter, flour, sugar, cocoa powder, baking powder and salt in a large mixing bowl. Mix the ingredients using an electric whisk until a sandy consistency is achieved. Lightly beat the milk and eggs in a jug. With the whisk going, gradually pour half of the milk and eggs into the bowl and mix thoroughly to combine. When the batter is smooth add the remaining liquid and continue to mix until combined. Stir the banana in.
Divide the batter equally among the 24 cupcake cases and bake for 20-25 minutes, or until the sponge bounces back when lightly touched. Leave to cool for 5 minutes in the tin, then place on a wire rack.
For the buttercream, use an electric whisk to beat together the icing sugar and butter until well combined. Gradually add the milk while mixing on a slow speed to make a smooth icing. Add the dulce de leche and mix thoroughly. Turn up the speed and beat until for a further 5-10 minutes, or until the icing is light and fluffy. Transfer to a piping bag fitted with a large star nozzle.
Once the cupcakes are cool, use a sharp knife to make a hollow in the centre of each cupcake, keep the cut-out piece of sponge. Put a teaspoonful of the dulce de leche into each hollow. Replace the cut-out piece of sponge, trimming to fit and pressing down gently to ensure that the top is level.
Pipe a swirl of buttercream on top of each cupcake and decorate with a little sea salt and pieces of fudge and dark chocolate.