In the bowl of your stand mixer, add in all dough ingredients with the exception of the butter. Mix with dough hook until a rough shaggy dough starts forming.
Add in half of the soft butter, letting it be fully incorporated before adding the rest.
Let dough mix on low (on my kitchenaid it is speed 2) for about 20 minutes. I personally take the dough out about halfway through and knead it a few times by hand, do a windowpane test and put back in the mixer.
Place dough in a lightly greased bowl and let rise in a warm spot for about 2 hours. Dough will have doubled in size.
Prepare a loaf pan with parchment or greasing.
Roll dough out to about 12x16 inches. Spread mascarpone in a thin layer over the dough, leaving about a ½ inch border. Spread lemon curd on top of mascarpone.
Starting on the short side, roll dough up into a log.
Cut dough down the center, so you have two pieces the same length as the original roll, with the inner layers exposed.
Take one piece of the dough and cross over the other creating an X. Twist the two pieces together on either end, then place in prepared loaf pan. Cover, and let rise for about 1 hour.
Preheat your oven towards the end of your proof time to 375°F
Bake babka for about 45 minutes on center rack. Check after about 20 to make sure the top is not burning. Due to the exposed curd, some points are likely to brown very quickly. Cover with tin foil for the remainder of bake if the top begins to get too brown.
Bread is done when thermometer inserted into center reads 190°F
Let cool in pan for about 5-10 minutes and then move to a cooling rack. Let bread cool for at least an hour before cutting into it.