Prepare the Dough
In a stand mixer bowl, combine flour, sugar, yeast, and salt. Add in the eggs and warm milk. Mix on low until a rough dough forms, then increase to medium speed. Add butter, one tablespoon at a time, until fully incorporated. Continue kneading for 10–12 minutes, until the dough is smooth and elastic.
Let It Rise
Place dough in a lightly greased bowl. Cover and let rise in a warm spot for 1–2 hours or until doubled in size.
Roll and Fill
Punch down dough. Roll out into a 10x16-inch rectangle on a lightly floured surface. Spread jam evenly over the surface, leaving a 1/2-inch border. (If using cornstarch, mix it into the jam before spreading.)
Shape the Loaf
Tightly roll the dough into a log from the long side. Use a sharp knife to slice the log lengthwise. Twist the two halves together with the cut sides facing up. Gently place into a greased 9x5-inch loaf pan.
Second Rise
Cover loosely and let rise for 45–60 minutes, until the loaf looks puffy.
Bake
Preheat oven to 350°F (175°C). Brush loaf with egg wash and sprinkle with sugar. Bake for 35–40 minutes, or until golden brown and cooked through (internal temp should be 190°F).
Cool and Enjoy
Let cool in the pan for 10 minutes, then transfer to a wire rack. Slice and serve warm or at room temperature.