For the Dough:
Heat the milk until the temperature is between 95°F - 115°F. Add the 1 tablespoon of sugar, stir until dissolved. Remove from heat. Sprinkle the yeast over the warm milk and allow the yeast to rest for 5 - 10 minutes or until it bubbles and thickens.
In the bowl of a stand mixer add the flour, eggs, vanilla, salt, 1/2 cup of sugar and melted butter. Using the dough hook attachment of a stand mixer, set the mixer on low and mix until well incorporated, 2 - 3 minutes.
Add in the yeast mixture, continue to mix on low until it is incorporated, then turn up the speed to medium and continue to mix until the dough pulls away from the sides, 5 - 10 minutes. You may need to scrape the dough down the sides to help. This may seem to take forever, but don't give up, it'll come together into a smooth, elastic texture.
Lightly butter or grease a large, clean bowl. Transfer the dough, rolling it around to coat dough on all sides. Cover with a clean cloth and let rise for about 1.5 - 2 hours or until doubled in volume.
To Assemble the Babka:
Line a 9" x 5" loaf pan with parchment paper. Transfer the dough to a lightly floured surface, punch down, divide into 2 portions, cover one to prevent it drying out while working with the other. Roll into approximately about an 12 - 18 rectangle.
Spread 1/3 cup of peanut butter over dough, leaving about 1/2 inch border along one short side. Spread 1/2 cup of your favorite jam or jelly over the peanut butter and scatter about 1/2 cup of roughly chopped roasted peanuts over the jelly, if desired.
Starting with the filling covered short end, steadily and evenly roll it up, sealing the edge. Rolling for the short side gives you more layers. Using a sharp knife, cut dough in half, lengthwise. Turn dough cut side up and place beside each other.
Starting in the middle of the dough, twist dough keeping the cut side up and seal/pinch dough to seal at end. Repeat from middle to end for other side. Very carefully transfer dough to the prepared, parchment lined loaf pan.
Cover loosely and let rise, about 1 - 1.5 hours, or until the dough has risen to the top of or above the loaf pan. Preheat oven to 350°F. Uncover babka dough, bake for a total 45 - 55 minutes. During the last 15 - 20 minutes of cooking, loosely cover the babka with foil, to prevent over browning. Babka should be golden brown.
If using the glaze: While the babka is baking, in a small saucepan, bring the water and sugar to a boil, stirring until sugar has dissolved. Set aside. Once babka is baked, remove from oven. Drizzle half of sugar glaze over each babka while the bread is still warm and in pans. Allow babka to cool in pan for 10 minutes, remove and peel away the parchment, transfer to a cooling rack to completely cool.