Preheat the oven 180 °C (356 °F). Peel the skin of 350 g Japanese sweet potato (satsumaimo) 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.)
Mash while it's still hot until no lumps remain.
Add 30 g sugar, 25 g unsalted butter, 1 ½ tsp honey, ½ tsp mirin, and ¼ tsp vanilla essence to the mashed potato. Mix until incorporated, then add 1 ½ tbsp heavy cream a little at a time, mixing with a spatula until incorporated. If you feel the mixture is becoming too soft, you don't need to add everything.
Bonus step: Scrape the mixture through a fine-mesh sieve for an extra smooth result.
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 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 for a few minutes, then serve them warm or chill in the refrigerator and serve later. Enjoy!
Notes
This recipe uses 1 medium size sweet potato (approx 350g), but ingredient quantities should be adjusted depending on the weight of the potato. For small sweet potato (250g/makes 4): 20g sugar, 1 tsp honey, ⅛ mirin, 16g butter, 1 tbsp cream, ⅛ vanilla. For large sweet potato (450-500g/makes 8): 40g sugar, 2 tsp honey, ¼ tsp mirin, 32g butter, 2 tbsp cream, ¼ tsp vanilla.Serve warm or chill in the fridge. Store at room temperature if you're eating them the same day (5-6 hours), or refrigerate and consume within 2-3 days.To freeze, wrap individually with plastic wrap and store in an airtight container. Defrost at room temperature before eating. If you prefer them hot, reheat in a toaster oven or microwave for about 20-30 seconds.