Preheat the oven to 180°C and line two 12-hole cupcake tins with paper cases.
Whisk the butter, flour, sugar, cocoa powder, baking powder, and salt until they reach a sandy consistency.
Beat the milk and eggs together in a jug.
Gradually whisk half the liquid into the dry ingredients and mix until smooth.
Add the remaining liquid, mix well, and stir in the mashed banana.
Divide the batter among the cases and bake for 20 to 25 minutes until the sponge bounces back.
Cool the cupcakes in the tin for 5 minutes before moving them to a wire rack.
Whisk the icing sugar and butter together until well combined.
Slowly mix in the milk and dulce de leche until smooth.
Beat the icing on high speed for 5 to 10 minutes until light and fluffy.
Transfer the buttercream into a piping bag with a star nozzle.
Cut a small hollow in the centre of each cooled cupcake and fill with a teaspoon of dulce de leche.
Replace the trimmed sponge tops and pipe a swirl of buttercream on each.
Decorate with sea salt, fudge pieces, and grated dark chocolate.