Preheat the oven 180 °C (356 °F). Peel the skin of 350 g Japanese sweet potato(s) and cut it into slices approximately 1cm thick. (Cut larger pieces in half.)
Place the pieces in a bowl of cold water with a pinch of salt and soak for 5 minutes. Drain the water and wash the pieces with cold running water to remove any excess starch.
Transfer the sweet potato to a microwavable bowl and cover with plastic wrap. Microwave for 5-6 minutes at 600W, mix halfway through to ensure even cooking. (Be careful of the steam when removing the plastic wrap.)
Allow to cool for a few minutes then mash while it's still hot/warm.
Add 30 g caster sugar, 1 ½ tsp honey, 25 g unsalted butter, 1 ½ tbsp heavy cream, ¼ tsp vanilla essence and ½ tsp mirin to the bowl and mix everything together with a spatula.
Transfer the mixture to a sieve and use the spatula to work it through to make it smooth.
Divide the mixture into equal 6 pieces and roll them into oval shapes.
Place them in flattened cupcake cases on a baking sheet, then whisk 1 egg yolk in a small bowl and generously brush the tops and sides until fully coated.
Sprinkle the tops with ¼ tsp black sesame seeds and bake for 15 minutes or until the tops are lightly browned. (Alternatively, use a toaster oven or grill and cook until browned on top.)
Cool on a wire rack.
Enjoy warm or chill in the refrigerator!