In a bowl of a standing mixer, mix the warm milk, 2 tbsp of sugar, and yeast and let sit for 20-25 minutes, until the mixture is frothy. That is a sign that the yeast has been activated.
In a large bowl, sift together the flour, remaining sugar, nutmeg if using, and salt. Add the mixture into the yeast mixture and with the paddle attachment, mix on low.
In a medium bowl mix together the sour cream, eggs, and vanilla then add it to the flour-yeast mixture.
Drizzle in the melted butter while the mixer is on low and mix until you have a shaggy mass.
Replace the paddle attachment with the dough attachment then mix until the dough is smooth, and the sides of the bowl are clean. This can take anything between 10-15 minutes.
Stop the mixer, spray the dough with a cooking spray then cover with a plastic wrap, and let rise until double in size about 2-3 hours.
Using a hand mixer, beat butter and sugar until incorporated.
Melt the chocolate, add to the butter mixture following the cocoa butter and mix to incorporate.
Cream butter with sugar and honey until smooth.
Add flour and nuts and mix until a pea-sized lumps form.
Line the bottom of the 2 8" loaf pan with parchment paper and grease the sides.
On a lightly floured surface, divide the babka dough into 3 equal parts.
Roll the first part into 12" wide and about twice the length of your pan.
Spread the dough with chocolate filling, leaving an uncoated band in the center and about 1"-2" from the bottom edge.
Sprinkle with chopped nuts or cake crumbs.
Starting with the edges furthest from you roll the dough towards you into a tight log and seal the bottom.
Fold the dough into half and twist it 3-4 times into a spiral.
Place the loaf into your prepared pan, brush with egg wash and sprinkle with streusel.
Let rise for about 2-3 hours.
Preheat oven to 350F and bake for 45-50 minutes, until the loaf is a rich brown color, and a thermometer reads 190-200F when inserted to the center of the cakes.
Remove from oven and let cool for about 10-15 minutes before removing from pan and slicing.