Make the Dough: Dissolve the yeast and sugar in the warm milk. In a mixer, combine the flour and the yeast mixture. Knead until a dough forms, add the butter, and knead until smooth. Let the dough rise in a warm place for 1 hour until doubled in size.

Prepare the Filling: Ensure your leftover Japanese curry is completely chilled and extremely thick (almost solid). If it is runny, it will burst out of the bread during frying.

Shape and Stuff: Punch the dough down and divide it into 8 equal balls. Flatten each ball into a 4-inch circle. Place a generous spoonful of the cold, thick curry in the center. Pinch the edges of the dough together incredibly tightly to seal the curry entirely. Shape into an oval.

Bread the Dough: Dip each stuffed dough oval completely into the beaten egg, then roll it firmly in the Panko breadcrumbs until heavily coated. Let them rest on a tray for a 20-minute second rise.

Deep Fry: Heat vegetable oil to 320°F (160°C). Carefully drop the breaded dough into the oil. Fry slowly for 3 to 4 minutes per side until the bread puffs up massively and turns a deep, dark golden brown. Drain on a wire rack and serve warm.