Mix the yeast with ½ teaspoon sugar and warm water; set aside until doubled in volume. Beat 2 ¾ cups flour, ½ cup sugar, egg, egg yolk, walnut extract, butter and fermented yeast in a large mixer bowl; gradually add evaporated milk. Mix together until smooth, elastic dough that does not stick to your fingers forms. Place in a large oiled bowl; cover with plastic wrap. Let rest in a warm place until doubled in size.