Heat your whole milk in the microwave until it reaches 115 degrees. Stir in the sugar, vanilla extract, and the yeast. Let the mixture sit and bloom for 5-10 minutes, or until it's bubbly and frothy. In a large bowl, or the bowl of a stand mixer, stir together the eggs and soft butter- it will be a bit lumpy, but that's ok!  Add the yeast mixture into the egg mixture, and stir to combine. Add in 3 cups of flour and the salt, and knead together until a dough forms, using the dough hook on a stand mixer. If the dough is super sticky and wet still, add a few more tablespoons of flour until it's pulling away from the sides of the bowl. It should be very soft still, but won't stick to your fingers. Knead the dough for 5-8 minutes, or until it's very elastic and smooth. Place the dough back in your bowl and cover. Let the dough rise until it's almost doubled in size, which takes about 30-45 minutes for me.

In a small bowl, stir together the brown sugar, cinnamon, salt, and apple butter until combined. Grease an 8-inch loaf pan and set aside.

Once your dough has risen, roll it out on a lightly floured surface into a large rectangle that is about 1/4 inch thick. Spread your filling evenly all over the dough, all the way to the edges. Roll the dough up tightly into a log from the long sides. Using a very sharp knife, slice the log in half length-wise. Then, with the cut sides up, twist the dough around itself. Fold the whole twisted log in half, and place it into your prepared loaf pan. This process can be a little messy since the apple butter is a very soft filling, but don't worry! It doesn't have to be perfect.

Cover it in plastic and let the dough rest for about 20 minutes while you preheat your oven to 375 degrees. Remove the plastic wrap and bake the babka for 40-45 minutes. An instant-read thermometer should show in the inside of the babka at about 175 degrees once it's done.

In a small sauce pot, stir together the water, sugar, and salt, and bring the mixture to a boil over medium heat. Remove from the heat, and cool for 10 minutes. Stir in the vanilla extract. When your babka comes out of the oven, immediately brush the top generously with your syrup mixture. Cool the babka completely in the pan, and then turn it out to slice and serve!