In a large bowl, whisk together the flour, salt and baking powder.

Cut the cold butter into small pats and add to the flour.  With your fingers, mush the butter and flour together until you get a crumbly flour/butter mixture - like the picture above.  You'll have fairly large chunks of butter, but you shouldn't have any loose flour on the bottom of the bowl.

Stir in the sour cream.

On a well-floured surface (I use a large wooden board with plenty of flour), turn out the dough and give it a few kneads - not more than 4 or 5 (kneading will make the dough tough).  Pat it out to a rectangle about the size of a half piece of paper.  Flour the top and roll it out until it's about double that size.  Fold it up like a letter, then fold the letter in half again.  You should have a piece of dough about the same size you started with.  Repeat that whole process again.  (This layering and folding is what gives you a flaky crust).  Flour the surfaces well and wrap the whole thing in plastic wrap and put it in the fridge.  It needs to rest and cool down a bit - at least 30 minutes.

While the dough is cooling, make your filling.   I find frozen berries are easier to work with, but you can use fresh.  Put the berries in a sauce pan.  Add the sugar, vanilla and cinnamon and stir constantly until the mixture is just warm.  Mix the corn starch with a little water (3 Tbl or so) until it's suspended.  Quickly pour this mixture into the berries while stirring.  Continue to heat just until the mixture starts to thicken - about 5 minutes or so.  Remove from heat and cool for a while.

In a small bowl, lightly beat one egg and set aside.  You'll use this to seal the turnovers.

Bake the turnovers in a 400 F oven for about 25 minutes, or until golden brown.