Preheat your oven to 180°C (355°F). Grease a square tin with butter and line the bottom with baking paper.

Combine the unsalted butter and milk in a microwave-safe bowl. Heat and stir until melted, then let it cool slightly.

In a large mixing bowl, sift together the cake flour, baking powder, and salt. Whisk until well combined.

Whisk the eggs and egg whites in another bowl until frothy, then gradually add the caster sugar while whisking until pale and fluffy.

Fold the dry ingredients into the egg mixture, then add the cooled butter and milk, stirring gently until just combined.

Pour the batter into the prepared tin and bake for approximately 30 minutes. Cool completely, then chill for 1 hour.

Prepare the coating by mixing powdered sugar with boiling water, melted butter, and vanilla extract until smooth.

Slice the chilled cake in half horizontally. Spread raspberry jam on the bottom half and then the whipped cream filling. Place the top half back on and chill again for 30 minutes.

Cut the cake into cubes, dip each in the coating, and roll in sprinkles before serving.