Dissolve yeast and 1/4 teaspoon sugar in milk. Let stand 5 minutes.
Lightly spoon flour into dry measuring cups; level with a knife. Place 2 1/2 cups flour and remaining sugar in a large bowl; make a well in center of mixture. Add vanilla to milk mixture. Pour milk mixture into well. Beat at medium speed with a mixer just until combined. Add 1 egg, egg yolk, and salt; beat just until combined. Gradually beat in butter. Turn dough out onto a work surface. Knead until smooth (about 8 minutes), adding enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 1/2 hours or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down; cover and let rest 5 minutes.
Divide dough into 16 equal portions; shape portions into balls. Make an indentation in each ball, and tuck about 10 chocolate chips in each indentation. Pull dough over chips, and pinch to enclose chocolate in center of each ball. Place rolls, seam sides down, in muffin cups coated with cooking spray. Cover and let rise 45 minutes or until doubled in size.
Preheat oven to 350°.
Lightly beat remaining egg; brush tops of rolls with egg. Bake at 350° for 15 minutes or until golden.
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