Make a flour paste: Whisk water and flour together in small bowl until no lumps remain. Microwave, whisking every 25 seconds, until mixture thickens to stiff, smooth, pudding-like consistency that forms mound when dropped from end of whisk into bowl, 50 to 75 seconds.

In bowl of stand mixer, whisk flour paste and milk together until smooth. Add egg and yolk and whisk until incorporated. Add flour and yeast. Fit stand mixer with dough hook and mix on low speed until all flour is moistened, 1 to 2 minutes. Let stand for 15 minutes. Add sugar and salt and mix on medium-low speed for 5 minutes. Stop mixer and add butter. Continue to mix on medium-low speed for 5 minutes longer, scraping down dough hook and sides of bowl halfway through (dough will stick to bottom of bowl).

Transfer dough to lightly floured counter. Knead briefly to form ball and transfer seam side down to lightly greased bowl; lightly coat surface of dough with vegetable oil spray and cover bowl with plastic wrap. Let dough rise until just doubled in volume, 40 minutes to 1 hour.

While dough rises, grease 13 by 9-inch metal baking pan. Whisk melted butter, brown sugar, granulated sugar, corn syrup, and salt together in medium bowl until smooth. Add water and whisk until incorporated. Pour mixture into prepared pan and tilt pan to cover bottom. Sprinkle evenly with pecans, if using.

Combine sugar and cinnamon in small bowl and mix until thoroughly combined; set aside.

Turn out dough onto lightly floured counter. Press dough gently but firmly to expel air. Working from center toward edge, pat and stretch dough to form 18 by 15-inch rectangle with long edge nearest you. Sprinkle filling over dough, leaving 1-inch border along top edge; smooth filling into even layer with your hand, then gently press mixture into dough to adhere.

Beginning with long edge nearest you, roll dough into cylinder, taking care not to roll too tightly. Pinch seam to seal and roll cylinder seam side down. Mark gently with knife to create 12 equal portions. To slice, hold strand of dental floss taut and slide underneath cylinder, stopping at first mark. Cross ends of floss over each other and pull. Slice cylinder into 12 portions and transfer, cut sides down, to prepared baking pan. (I use a serrated knife because the dental floss, while piquing my curiosity, sounds too complicated). Cover tightly with plastic wrap and let rise until buns are puffy and touching one another, 40 minutes to 1 hour. (Buns may be refrigerated immediately after shaping for up to 14 hours. To bake, remove baking pan from refrigerator and let sit until buns are puffy and touching one another, 1 to 1 1/2 hours.) Meanwhile, adjust oven racks to lowest and lower-middle positions. Place rimmed baking sheet on lower rack to catch any drips and heat oven to 375 degrees.

Bake buns on upper rack until golden brown, about 20 minutes. Tent with aluminum foil and bake until center of dough registers at least 200 degrees, 10 to 15 minutes longer. Let buns cool in pan on wire rack for 5 minutes. Place rimmed baking sheet over buns and carefully invert. Remove pan and let buns cool for 5 minutes. Using spoon, scoop any glaze on baking sheet onto buns. Let cool for at least 10 minutes longer before serving.