Egg whites and butter spread

This spread can be used as a cake filling.

1 cup (200g) sugar
1/4 cup (60 ml) water
4 egg whites
1/4 cup (50g) sugar
250g butter, softened

Whisk egg whites and ¼ cup sugar with an electric mixer until it forms stiff peaks.

Place water and 1 cup sugar in a saucepan.
Heat and bring to the boil. Keep boiling until syrup reaches the stage of hard ball.

Remove from heat and pour syrup in a thin stream into egg whites.
Continue beating until all syrup is used and frosting has cooled almost completely.

Fold in softened butter.