Dissolve 1 teaspoon granulated sugar and yeast in warm milk in the bowl of a stand mixer; let stand 5 minutes.
Stir in 6 tablespoons granulated sugar, vanilla extract, 1/4 teaspoon salt, and egg yolk. Add 6 ounces (about 1 1/3 cups) all purpose flour and the bread flour to milk mixture; beat with dough hook attachment at medium speed until well blended (about 2 minutes).
Add 5 tablespoons butter, beating until well blended. Scrape dough out onto a floured surface. The dough will be very sticky. Knead until smooth and elastic (about 10 minutes); add 1.5 ounces (about 1/3 cup) all purpose flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will be very soft). **I used my mixer to knead the dough. I just set it to medium speed and used the dough hook to knead for 10 minutes. I added the 1.5 ounces of flour, 1 tablespoon at a time, to the dough every minute or so, until it was completely incorporated.
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 dough rest 5 minutes. While the dough is rising, prepare the filling.