Cook the potato in a large saucepan of salted, boiling water until tender; usually 15-20 minutes. Drain once ready and let the potatoes steam-off a little before mashing with the melted butter and seasoning until smooth.
Add the egg yolks when the mixture is still warm and beat well until incorporated. Spoon the mash mixture into a piping bag fitted with a 2-3 cm in diameter star-shaped nozzle.
Pre-heat the grill to hot. Pipe blobs of the potato mixture well spaced apart onto baking trays. Grill for a few minutes until the croquettes are golden and crisp on the outside.
Remove from the grill when ready and season with a little of the flaked sea salt before serving.