first dough;take a bowl,bread flour,100g,water,100ml,instant yeast,1.5g.Mix well.cover with plastic wrap,rest the dough for 60min.Main dough;in the mixer bowl bread flour,300g,milk powder,15g,egg,1,sugar,35g,milk,160ml,salt,3g,instant yeast,8g.Add the first dough,knead the dough for 7minadd butter,25g,knead the dough for 4min.Rest the dough for 60min.Separate the dough into 8pieces.Make balls.Cover the dough.Rest the dough for 15min.Make the sandwich buns; pan size 28x28,rest the dough for 50min,brush with milk,bake in a preheated oven 190C for 20min