Make the Dough

1. Prepare the Bowl:
   • Grease a large bowl with butter or oil.
   • Tip: A slick bowl prevents dough sticking during rising.
2. Mix Wet Ingredients:
   • In a large liquid measuring cup, whisk 4 large eggs, ¾ cup warm whole milk (100-110°F), and ¼ cup honey until combined.
   • Tip: Ensure milk is warm but not hot to avoid killing yeast or cooking eggs.
3. Combine Dry Ingredients:
   • In a stand mixer fitted with a paddle attachment, mix 4 cups (568 grams) all-purpose flour, 2¼ teaspoons instant yeast, and 2 teaspoons salt on low speed to combine.
   • Tip: Stir briefly to distribute yeast evenly.
4. Form the Dough:
   • Add the egg mixture to the flour mixture and mix on low until combined.
   • With the mixer on low, add 10 tablespoons (142 grams) butter, one piece at a time, waiting until each is incorporated.
   • Increase speed to medium and beat for 1 minute until butter is fully integrated and dough is sticky.
   • Tip: Scrape the bowl as needed; dough will be very soft and sticky.
5. First Rise with Folds:
   • Transfer dough to the greased bowl using a spatula (it’s sticky).
   • Cover with plastic wrap and let rise for 30 minutes.
   • Perform a series of folds: gently pull dough up and fold over itself, rotate bowl, and repeat 6-8 times total.
   • Re-cover and let rise 30 minutes. Repeat folding 3 more times (2 hours total, 4 foldings).
   • Tip: Folding strengthens dough structure; wet hands prevent sticking.
6. Refrigerate Dough:
   • Tightly cover with plastic wrap and refrigerate overnight or up to 72 hours.
   • Tip: Longer refrigeration (up to 3 days) deepens flavor; ensure a tight seal to prevent drying.

Make the Rhubarb Blackberry Jam

1. Cook the Jam:
   • In a medium saucepan, combine 2 cups (250 grams) rhubarb, 6 ounces (170 grams) blackberries, ½ cup (100 grams) granulated sugar (up to ¾ cup if berries are tart), a pinch of salt, and 1 split vanilla bean (seeds and pod).
   • Simmer over medium-low heat for 30-40 minutes, stirring often, until rhubarb and blackberries break down and jam clings to a wooden spoon.
   • Tip: Stir frequently to prevent sticking; test thickness by checking if jam coats the spoon.
2. Cool and Adjust:
   • Remove from heat, discard vanilla bean, and cool to room temperature.
   • Taste and add ½ teaspoon lemon juice at a time if needed to brighten flavor.
   • Refrigerate until ready to use (up to 1 week).
   • Tip: Cool completely to avoid melting butter in dough; store in an airtight container.

Make the Streusel

1. Mix Streusel:
   • In a stand mixer with paddle attachment, combine ⅔ cup (94 grams) all-purpose flour, ⅓ cup (66 grams) granulated sugar, ⅓ cup (66 grams) brown sugar, ½ cup (50 grams) almond flour (or rolled oats), ¼ teaspoon salt, and ¼ teaspoon cinnamon on low speed.
   • Add 6 tablespoons butter, one piece at a time, mixing on low until crumbly but cohesive.
   • Tip: Stop mixing when crumbs form; overmixing makes streusel dense. Refrigerate if not using immediately.

Make the Icing

1. Mix Icing:
   • In a small bowl, whisk 2 ounces (56 grams) cream cheese, 2 tablespoons milk, a pinch of salt, and ½ teaspoon vanilla extract until smooth.
   • Add 1 cup (113 grams) powdered sugar and whisk until smooth. Add more sugar (up to ¼ cup, 142 grams total) for thicker icing.
   • Stir in 2 tablespoons melted, cooled butter until combined.
   • Tip: Adjust sugar for drizzle consistency; cover and refrigerate if making ahead (whisk before using).

Assemble and Bake

1. Prepare the Pan:
   • Line a half sheet pan with parchment paper or generously grease a 10-inch round pan (e.g., Mauviel) with butter.
   • Tip: Parchment ensures easy release; grease round pans thoroughly for clean removal.
2. Shape the Dough:
   • On a lightly floured surface, fold cold dough over 3-4 times to deflate. Form into a ball and rest for 15-20 minutes.
   • Roll into a ¼-inch thick rectangle (about 12×16 inches).
   • Tip: Flour surface sparingly to prevent sticking; rest makes rolling easier.
3. Fill and Roll:
   • Brush dough with 2 tablespoons melted butter.
   • Spread ½ cup rhubarb-blackberry jam thinly over the dough, adding more if needed for a light layer (avoid excess to prevent mess).
   • Roll tightly from the short end into a log.
   • Tip: Spread jam evenly, leaving a ½-inch border; roll snugly to keep filling contained.
4. Cut and Arrange:
   • Using a sharp knife or scissors, cut the log into 12 equal pieces (~1-inch thick).
   • Place buns on the prepared pan, 1½-2 inches apart.
   • Tip: Cut gently to avoid squishing; space buns for even rising and baking.
5. Final Rise:
   • Loosely cover buns with plastic wrap and let rise for 1½-2 hours until nearly doubled. Alternatively, refrigerate overnight (8-12 hours), then let sit at room temperature for 45-60 minutes before baking.
   • Tip: Rise in a warm, draft-free spot (75-80°F); buns should look puffy and soft.
6. Bake:
   • Preheat oven to 350°F with a rack in the middle.
   • Generously sprinkle streusel over buns, gently pressing to adhere (reserve extra streusel for another use).
   • Bake for 22-27 minutes, until centers are set (spring back when poked) and tops are light golden brown.
   • Tip: Check at 20 minutes; tent with foil if streusel browns too quickly.
7. Cool and Ice:
   • Cool buns on the pan on a wire rack for 10-15 minutes.
   • Drizzle with icing using a spoon or piping bag.
   • Serve warm or at room temperature (makes 12 buns).
   • Tip: Ice while slightly warm for a glossy finish; serve extra icing on the side for dipping.

Cooking Tips:

• Dough Stickiness: Sticky dough is normal; use a spatula and minimal flour to handle.
• Jam Thickness: Ensure jam is thick to prevent leaking; refrigerate to firm up before spreading.
• Streusel Application: Apply generously but don’t overload; extra streusel can be baked separately as a crumble.
• Doneness Check: Buns should be golden and firm in the center; avoid overbaking to keep them soft.