Peel and cut the satsumaimo into 3 cm cubes. Place in a steamer basket over boiling water.

Steam for 18 to 20 minutes until a skewer slides through with no resistance.

Transfer to a mixing bowl and mash immediately while still hot, using a potato ricer or pressing through a fine-mesh sieve for the smoothest result.

Add the butter to the hot mash and stir until fully melted and combined.

Add the sugar and salt, then stir until dissolved, about 1 minute.

Let the mixture cool to warm, not hot, then mix in the heavy cream and egg yolk until the dough is smooth and holds its shape when pressed.

Divide the dough into 8 equal portions, about 70 g each.

Place each portion on a piece of plastic wrap, fold the wrap over, and shape into a smooth oval about 7 cm long.

Place the shaped cakes on a parchment-lined sheet pan and refrigerate for 15 minutes.

Heat the oven to 180 C / 355 F.

Whisk together the egg yolk and soy sauce in a small bowl until combined.

Brush the top and sides of each chilled cake with a thin layer of egg wash. Let sit for 2 minutes.

Using a small sharp knife, score a shallow crosshatch pattern on the top of each cake.

Apply a second coat of egg wash over the scored surface.

Bake on the middle rack for 22 to 25 minutes until the surface is deep golden brown and the crosshatch pattern is defined.

Remove from the oven and cool on the pan for at least 10 minutes before serving. The cakes firm up slightly as they cool.