Wash beans well and soak overnight with plenty of water. In the morning, boil until tender. Add more water if necessary to prevent burning. Stir occasionally. Drain. Use a blender to puree the beans. Put the pulp into a clean cotton bag to squeeze out excess water. In a heavy saucepan, mix beans, sugar, and salt. Boil under low heat, stirring constantly until very thick and smooth. An will burn easily. When An is ready, you can form a soft, moist ball with your hands.
Sift dry ingredients three times. Cream butter and sugar until fluffy and creamy. Add eggs, milk, and dry mixture. Blend well. Mixture should be soft, but not sticky. Amount of milk depends on the size of eggs. Knead well, and cut dough into size of large walnut. Flatten out in your palm and place small An ball in center of dough. Wrap dough around the An and pinch to seal. Place pinched side down on greased cookie sheet. Glaze with beaten egg yolk and soy sauce using pastry brush. Back in 375° oven for 12-13 minutes or until lightly brown.