Preheat your oven to 180°C (conventional) or 160°C (fan). Line a 12-hole muffin tin with colorful paper cases.

In a large mixing bowl, combine softened butter or margarine with banana milkshake powder, all-purpose flour, and eggs. Beat until smooth and creamy, about 2-3 minutes.

Scoop the batter evenly into each prepared muffin case, filling them about two-thirds full. Bake for 25-30 minutes or until golden brown.

Let the cupcakes cool in the tin for 5-10 minutes, then transfer them to a wire rack to cool completely.

Cream softened butter in a mixing bowl with half of the sifted icing sugar until fluffy and pale.

Gradually add the remaining icing sugar, banana milkshake powder, and milk to reach the desired consistency. Whip for an additional 2-3 minutes.

Once the cupcakes are cool, frost each with the buttercream icing using a piping bag or spatula.