In a bowl, dissolve yeast in warm water. Stir in flour, cover with plastic wrap and set in a warm place until foamy, about 10 minutes. (I like to place on the stove under the light).
For the dough:
In the bowl of an electric mixer, combine 2 cups flour, sugar and salt; whisk by hand until well blended. Transfer bowl to mixer and attach dough hook. With mixer on low speed, add egg and milk. Increase speed to medium and knead 5 minutes. Add butter and knead 5 minutes. Stop mixer and add sponge. continue kneading on medium speed until dough is smooth and elastic, 7 to 8 minutes, adding more flour if needed. Cover the bowl with a towel and let rise in a warm place until doubled, about 1 hour.
Butter 12 cupcake molds. Punch down dough, transfer to a lightly floured surface and knead in chocolate chips. Divide dough into 12 equal parts.
Roll on the table in a circular motion to give body to the brioche. With your finger, "saw" a small piece of the brioche in a back and forward motion. This forms a small lump which should remain attached to the body of the brioche.
Place the brioches into the buttered molds.and let rise in a warm place 45 minutes.
Brush tops of brioches with egg mixture. Bake until golden brown, 15 to 20 minutes. Remove from molds and let cool completely. Store brioches in a plastic bag to avoid drying out