In food processor, pulse together flour, sugar, and salt.
Sprinkle in chopped, cold butter and pulse until coarse crumbs begin to form.
Add sour cream, pulse until mixture begins to clump.
Pour dough out onto clean, well-floured surface and lightly dust with more flour (mixture will likely be very sticky).
Knead dough, adding flour as needed until it is cohesive and no longer sticky (you may need to add as much as ¼ cup flour).
Using your palm, flatten the dough on your clean surface.
Fold dough into itself, repeat flattening process (this helps encourage flaky layers in the crust) and then form dough into a disk, wrap tightly with clear wrap and place in fridge to chill for at least 30 minutes.
After 30 minutes, remove dough from refrigerator and preheat oven to 400F.
Remove clear wrap from dough and turn dough out onto lightly floured surface.
Use a rolling pin to roll dough until it is 1/8" or thinner.
Using a pizza cutter (or a knife and a straight-edge, such as a clean ruler), neatly cut dough int 4.5" squares and transfer to a parchment-paper-lined baking sheet (you will need to gather the scraps and re-roll them out to make all 10 turnovers).
Add your vanilla extract to your melted butter and stir. Use a pastry brush to brush the center of each square with butter (leave about a 1" perimeter unbrushed).
Evenly divide your finely chopped chocolate among the turnover squares, placing most of the chocolate in the center with decreasing amounts towards diagonal corners (don't go all the way to the end of the square, though).
Fold one opposite (without chocolate) corner of each square over to the other and use the tines of a fork to seal each turnover.
Brush each turnover with the lightly beaten egg, and combine your sugar and cinnamon in a small bowl.
Generously top each turnover with cinnamon/sugar mixture, and cut small slits across the top of each triangle to vent.
Bake on 400F for 20 minutes, or until turnovers are lightly golden brown.
Allow to cool on pan before enjoying (chocolate can be extremely hot!).