For the sponge:
- 2 tsp. active dry yeast
- 3 Tbs. lukewarm water
- 3 Tbs. all-purpose flour
For the dough:
- 1 3⁄4 cups all-purpose flour, plus more
- 3 Tbs. sugar
- 1⁄4 tsp. salt
- 1 egg
- 1⁄3 cup warm milk
- 4 Tbs. (1⁄2 stick) unsalted butter, cut into
pieces, at room temperature
- 1⁄2 cup semisweet chocolate chips
- 1 egg yolk, lightly beaten with 1 tsp. water
To make the dough, in the bowl of an electric mixer, combine the 1 3⁄4 cups flour, the sugar and salt and whisk by hand until well blended, about 1 minute. Transfer the bowl to the mixer and attach the dough hook. With the mixer on low speed, add the egg and milk. Increase the speed to medium and knead for 5 minutes. Add the butter and knead for 5 minutes. Stop the mixer and add the sponge. Continue kneading on medium speed until the dough is smooth and elastic, 7 to 10 minutes, adding more flour as needed. Cover the bowl with a towel and let rise in a warm place until doubled, about 1 hour.
Preheat an oven to 400ºF. Butter two 9-well silicone brioche baking molds and place on baking sheets.
Punch down the dough, transfer to a lightly floured surface and knead in the chocolate chips. Form the dough into 18 balls and place in the prepared wells. Let rise in a warm place for 45 minutes. Brush the tops of the brioches with the egg yolk mixture. Bake until golden brown, 15 to 20 minutes.
Immediately invert the molds onto wire racks and remove the brioches, using a toothpick to gently loosen them, if needed. Let cool completely before serving.