Beat yeast mixture with paddle attachment of electric mixer on low speed, adding eggs and yolk one at a time, beating well after each addition. Switch to dough hook. Add flour mixture and mix on low speed until well combined. Gradually mix in 1/2 cup butter, beating well after each addition on medium speed until fully incorporated, about 10 minutes. Dough will be very sticky. Cover and let rise at room temperature until doubled in size, about 1 hour.