In a small bowl or measuring cup, stir 1 envelope active dry yeast and 1 tsp granulated sugar in warm water (about 110F) until dissolved. Set aside and let stand until bubbly (about 10 minutes).
In a large bowl, stir ⅔ cup sugar and ¼ cup oil until fully combined.
Add 2 tsp salt, your yeast mixture, 1 cup warm water and 1 cup flour. Stir until incorporated. Proceed to add the rest of your flour 1/2 cup at a time combining well after each addition (see notes).
Once the dough starts pulling and sticking together in a ball, transfer it into a floured surface and knead until smooth and elastic (8-10 minutes). When in doubt, knead some more.
Grease a large bowl with oil. Place your dough in the greased bowl and turn to ensure the whole dough is coated. Cover with a damp cloth and allow to rise in a dry warm place (about 90F) until it doubles in size (about 1.5 hours).
Once the dough is ready, turn it into a lightly floured surface and knead slightly. Weigh the dough and divide into 18 pieces.
Flatten each piece into an oval/rectangle that's approximately 10 x 3 inches. Don’t flatten it too thin; go no thinner than 2mm.
Spread 1-2 heaping tablespoons of buttery filling (instructions follow) on the dough, leaving a half inch border all around.
Roll the dough and pinch at the seams to seal.
Roll in bread crumbs until full covered.
Place on lightly greased baking pans (you'll need 2) and cover with a damp cloth. Space evenly. Allow to rise a second time (about an hour).
Preheat your oven to 375F. Bake for 20-25 minutes or until the bread turns a light golden brown.
For the Spanish bread filling:
In a small bowl, combine ½ cup softened unsalted butter, ¾ cup breadcrumbs, ½ cup brown sugar and 1-3 tbsp milk (see notes) until combined.
If your dough is too wet or sticky, add more flour a little at a time until the dough becomes manageable.
When making your filling, start with 1 tbsp milk and add more until you achieve spreadable consistency.
Since you're using 2 pans, it would be a good idea to rotate the pans halfway through baking especially if your oven has hot spots. Check in and see. I use 2 half sheet pans that's 18x13 inches each.
See the post for the complete step-by-step photos and bread baking tips.