Combine lukewarm milk, instant yeast, and granulated sugar in a mixing bowl. Let sit for 5-10 minutes until foamy.

Mix in eggs, all-purpose flour, vanilla extract, and salt until a thick dough forms.

Gradually incorporate room temperature unsalted butter until the dough is glossy and smooth.

Shape the dough into a ball and place it in a lightly oiled bowl, turning to coat. Cover and let rise for 1-2 hours until doubled in size.

Gently punch down the dough, cover with plastic wrap, and refrigerate overnight for 8 to 16 hours.

Roll the risen dough into a 10x16 inch rectangle on a lightly floured surface.

Spread half of the softened cinnamon filling evenly over the dough. Roll into a tight log and chill for 15 minutes.

Cut the log lengthwise to expose the filling and twist both halves together.

Place the braided babka into greased loaf pans. Cover and let rise for 1-2 hours until nearly doubled.

Preheat oven to 350°F (180°C). Bake for 35-40 minutes until golden brown and a toothpick comes out clean.

Brush the baked babka with the sugar syrup immediately after removing from the oven and cool in pans for 20 minutes.