Filling: 600 g sweet red bean paste (smooth or chunky; store-bought, or homemade from 250 g dried adzuki beans)
Egg wash: 1 yolk + 1 tsp milk
Black sesame seeds (optional)
How to make it
Dough (10 min plus 30 min): Whisk the egg, sugar, honey, butter and milk. Mix in the flour, soda and salt to a soft, smooth dough. Wrap and rest 30 minutes.
Filling (5 min): Roll the red bean paste into 20 balls of about 30 g.
Wrap (20 min): Divide the dough into 20 pieces of about 15 g. Flatten each into a thin disc, set a bean ball in the middle and push the dough up and around to seal. Roll smooth, then flatten slightly into a puck.
Stamp (5 min): Press the top with a flower stamp or the back of a fork in a pattern. Brush with egg wash (and add a few sesame seeds).
Bake (12 to 15 min): Bake at 180C (350F) for 12 to 15 minutes until golden brown. Cool; they're even better the next day.
Tips
The dough should be much thinner than the filling: about one part dough to two parts bean.
Homemade paste: simmer soaked adzuki beans until soft, then cook down with 200 g sugar and a pinch of salt until thick.