Poke yams in several places with a fork, then cook them on high power in microwave oven until heated through and soft, about 20 minutes. When cooled, remove skins and place the yams in a large bowl.
Beat yams until smooth. Then beat in one egg at a time, blending after each addition. Stir in 1/4 cup brown sugar, 1/8 cup melted butter, salt, cinnamon, allspice and nutmeg; beat until the mixture is light and fluffy.
Transfer mixture to an ungreased 3 quart casserole dish. Arrange the pecans in a single layer on top. Sprinkle with remaining brown sugar and drizzle 1/4 cup melted butter over the top.
Bake for 25 minutes in the preheated oven, or until browned and bubbly.