Make the rye soaker: In a small bowl, mix 1/2 cup rye flour, 1/3 cup warm water, and vinegar. Cover and let rest 30–60 minutes. This softens the rye and boosts flavor.
Mix the dry ingredients: In a large bowl, combine bread flour, 1 1/2 cups rye flour, instant yeast, salt, caraway seeds, sugar, and vital wheat gluten (if using).
Add the wet ingredients: Stir in the rye soaker, warm water, oil or butter, and molasses (if using).
Mix until a shaggy dough forms. If it’s too dry, add water 1 tablespoon at a time.
Knead the dough: Knead by hand for 8–10 minutes or in a stand mixer on medium-low for 5–6 minutes. The dough will be slightly tacky but should pull away from the bowl.
Dust lightly with rye flour if sticky, but avoid over-flouring.
First rise: Shape into a ball and place in a lightly oiled bowl. Cover and let rise in a warm spot until puffy and nearly doubled, about 60–90 minutes.
Shape the loaf: Turn the dough onto a lightly floured surface. Gently press into a rectangle, then roll it up tightly into a loaf, pinching the seam closed.
Place seam-side down on a parchment-lined baking sheet or into a greased loaf pan.
Second rise: Cover and let rise again until the loaf has risen by about 60–75% and springs back slowly when poked, 45–75 minutes depending on room temperature.
Preheat the oven: Heat to 425°F (220°C). Place a small oven-safe pan on the lower rack for steam (optional, improves crust).
Make the cornstarch glaze: In a small saucepan, whisk cornstarch with 1 tablespoon water to make a slurry. Add remaining water, bring to a brief boil, then cool.
Brush a thin layer over the risen loaf. Sprinkle extra caraway seeds on top if you like.
Score and bake: Use a sharp knife or lame to score 3 diagonal slashes across the top. Pour 1/2 cup hot water into the preheated pan for steam (if using).
Bake 15 minutes at 425°F, then reduce to 375°F (190°C) and bake 18–25 minutes more, until deep brown and the internal temperature reaches about 200–205°F (93–96°C).
Finish with glaze: Brush a light second coat of cornstarch glaze on the hot loaf for shine. Cool completely on a rack—at least 1 hour—before slicing for clean, thin slices.