Preheat oven to 180ºC conventional/160ºC fan bake. Grease a 20cm square cake tin and line with baking paper.

Using an electric beater, cream butter, sugar and vanilla together until pale and fluffy.

Add eggs one at a time, beating well after each addition.

Sift in half the flour and baking powder and gently fold to combine.

Stir in half the milk. Repeat with the remaining flour, baking powder and milk.

Pour mixture into prepared tin. Bake for 25 minutes, until a skewer inserted into the centre comes out clean.

Leave to cool in tin for 5 minutes, then transfer to a wire rack to cool completely.

Once cool, trim the edges off and cut into 9 even cubes. Freeze for 1 hour, prior to icing.

In a medium bowl, dissolve jelly crystals in the boiling water.

Whisk in butter and icing sugar until well combined and smooth.

Place half the coconut in a separate bowl or pan.

Line a large tray with baking paper.

Place a piece of sponge cake in the icing and quickly roll to coat (use 2 forks).

Transfer to the coconut and roll to coat all over. Place on tray.

Repeat with remaining sponges, adding more coconut as needed.

Leave at room temperature for 1 to 2 hours, or until set.

Whip cream and icing sugar, if using, to soft peaks.

Transfer to a large piping bag fitted with a large open star piping tip.

Slice through the top of each lamington in the centre, to about halfway down.

Fill with whipped cream and top with a dollop of jam.