In a bowl or glass measuring cup, gently stir the yeast into the warm water. Let sit for 5 to 10 minutes, until the yeast almost doubles in size. Meanwhile, make the filling.
Make the filling
In a large bowl, mix together the softened butter, light brown sugar, ½ cup breadcrumbs, and vanilla. Set aside.
Make the bread
In a separate large bowl, whisk together the milk (or cream), granulated sugar, melted & cooled butter, egg yolks, salt, and yeast.
Gradually whisk in the flours, alternating between the bread flour and the all-purpose flour, until the mixture forms a dough ball.
On a lightly floured surface, knead the dough for about 10 minutes, just until the dough becomes smooth and elastic. Lightly grease the inside of a large glass bowl with olive oil or butter and place the dough in the bowl. Cover with a towel or plastic wrap and allow to rise in a dark, warm place for about an hour. The dough should be double in size.
Return the dough to the floured surface, and divide into four even dough balls. Then gently roll each ball out to form a log, about 6" long by 2" wide.
Use a pastry bench or pastry cutter to cut each log into 5 equal pieces and roll each into a ball (see Notes). Use a rolling pin to roll each of those pieces flat into an oval shape about 6" long by 3½" wide.
Place one tablespoon of filling in the center of each oval, leaving about ¼" space around the edges. Then roll them up and secure the tip by pressing gently.
Gently press the top of each into the shallow bowl of the remaining ½ cup breadcrumbs. Cover with a damp towel and allow to rise for a second time, about 30 minutes, just until the dough has doubled in size. Meanwhile preheat the oven to 325°F and line 2 baking sheets with parchment paper.
When the rolls have doubled in size, place them on the prepared baking sheets a couple of inches apart. Bake for 20 to 25 minutes, rotating and switching the trays halfway through for even baking. The tops should be just lightly golden brown. Allow to cool on the baking sheet for 5 minutes before serving.
This recipe calls for a mix of all-purpose flour and bread flour, for a more classic texture. You can use all AP flour instead if you don't have bread flour.For accuracy, use a kitchen scale to weigh the dough balls; each should weigh right around 50 grams.