Transfer the yolks to a bowl and mash with a fork. Pass the mashed yolks through a fine-mesh sieve to achieve a smooth texture. Add the softened butter, sugar, milk powder, cornstarch, salt, and heavy cream. Mix until smooth and well combined.

Brush the edges of a slice of bread with egg wash. Place a generous spoonful of the salted egg yolk filling in the centre. Top with a second slice of bread and gently press around the edges to seal. Repeat with the remaining bread and filling.

Bake in a preheated 180°C (350°F) oven for 12 minutes, or until the bread is golden brown and crisp.

Serve immediately while warm, allowing the salted egg filling to remain soft and creamy.