First, crumble the yeast into the mixing bowl along with the milk, sugar, and salt. Warm for 2 min / 37°C / Speed 2.
Add the flour and cornstarch, then set to 8 min / Dough Kneading Mode. After the first 3 minutes, gradually add the egg and butter pieces through the lid opening, and let it finish kneading.
Transfer the dough to a bowl and let it rise in a warm spot for 1 hour. Tip: Make sure to keep yeast dough away from cold drafts!
In the meantime, prepare the simple syrup. Add the sugar and water to the cleaned mixing bowl and bring to a boil for 6 min / 100°C / Speed 1. Transfer the syrup to a jar or small container and set aside.
Grease two loaf pans (approx. 4 x 9 inches / 10 x 23 cm) or line them with parchment paper (wringing it out first makes it so much easier to fit!). If using silicone loaf pans, no greasing or lining is necessary.
Once risen, divide the dough in half. Working with one piece at a time on a silicone mat or lightly floured surface, roll it out into a rectangle about 12 x 16 inches (30 x 40 cm). Spread with jam, leaving a 1/2-inch (1-2 cm) border around the edges. Roll up tightly starting from the short end, and place seam-side down.
Now it's time to shape your babka! A traditional babka consists of two twisted strands.
Using a sharp knife, slice the dough log lengthwise down the middle, leaving one top end intact. Flip both strands so the open, layered sides are facing up. Twist the two strands over each other in a simple spiral braid.
Pinch the open ends firmly together to seal.
Gently place the braided loaves into your prepared loaf pans, cover, and let them rise for another 1.5 hours.
Towards the end of the second rise, preheat your oven to 375°F (190°C) conventional heat. Bake both loaf pans side-by-side on a rack in the lower third of the oven for about 30 to 35 minutes, until the babkas are fully baked and beautifully golden and puffy.
Keep an eye on them while baking! Cover the babkas loosely with a sheet of aluminum foil halfway through so they don't get too dark on top.
Remove the babkas from the oven and cover them with a clean kitchen towel. After about 5 to 10 minutes, brush them with a generous coat of the simple syrup. (Seal and store any leftover syrup in the fridge for up to 6 weeks—it's fabulous for soaking cake layers or shaking up cocktails!)
Let the babkas cool in their pans until warm to the touch before carefully unmolding.