Instructions:
- Activate the Yeast: Gently warm milk, dissolve 1 tsp sugar, then whisk in yeast. Let sit until frothy (about 5–10 minutes).
- Mix the Dough: In a stand mixer bowl, combine remaining sugar, flour, and salt. Add the yeast mixture, beaten eggs, and vanilla. Mix on low speed.
- Knead and Enrich: Once mostly combined, increase speed to medium-low. Gradually add softened butter, one tablespoon at a time, ensuring each piece is incorporated before adding the next.
- First Proof: Knead until the dough is smooth and elastic (about 8–10 minutes). Place in a lightly oiled bowl, cover, and let rise in a warm spot until doubled in size (approx. 1.5–2 hours).
- Prepare the Topping: While the dough rises, prepare the streusel. Combine cold butter, sifted flour, and powdered sugar (and cocoa powder if making chocolate). Use your fingertips or a pastry blender to quickly cut the butter in until the mixture resembles coarse crumbs. Divide and gently mix in vanilla for the plain batch, if using. Chill thoroughly.
- Shape the Rolls: Gently punch down the risen dough. Divide into 12 equal pieces. Roll each piece into a tight ball. Place on lined baking sheets, leaving plenty of space between rolls.
- Second Proof (Resting): Cover the shaped rolls loosely and let them rest again for 30–45 minutes until visibly puffy.
- Prepare the Topping for Application: Take the chilled topping mixture. For the vanilla portion, gently press it together with your hands until it just barely holds a soft, pliable texture—it should still look crumbly. If too soft, chill briefly.
- Apply the Topping: Roll the topping mixture out to about 1/8 inch thickness between two sheets of parchment paper. Use a round cookie cutter (slightly larger than the bread roll) to cut out circles. Gently place one circle atop each proofed roll, pressing lightly so it adheres.
- Score the Shells: Using a dull knife or bench scraper, gently score a crosshatch pattern onto the topping circle, taking care not to cut deeply into the dough beneath.
- Bake: Preheat oven to 375°F (190°C). Bake for 18–22 minutes, rotating halfway, until the bread is golden brown underneath and the topping is set.
- Cool: Cool on wire racks.