These wonderful treats are the result of my efforts to replicate toaster pastries, which are a great vice of mine. I finally nailed them.
2 cups flour
1/2 tsp salt
1 cup cold butter, cut into 1/2" pieces (2 sticks)
1/2 cup yogurt or sour cream
2 - 3 cups cooled Apple Pie Filling
Whisk together flour and salt. Use a pastry cutter or food processor to cut the butter into the flour until it is evenly crumbly (the crumbs should be the size of small peas). Stir in the yogurt and then knead just until all the dough is moistened and sticks together. You can add a tsp or two of water if the dough is not sticking together after a couple minutes.Chill the dough in the fridge for an hour or overnight. This is important. It won't be flaky unless the butter starts out really cold.
Preheat oven to 400 degrees.
Once chilled, take the dough out and divide it into two pieces. On a well floured surface, roll out the first piece of dough into a large square. Roll as thin as you can get away with. Dusting more flour on the surface and rolling pin throughout the rolling process helps mitigate sticking. Picking the dough up, flipping and rotating it every couple rolls helps too. Once rolled out, cut the dough into roughly equal sized squares (see pic below). Don't worry about the rough edges, they will be taken care of later.
Place half of the dough squares on a cookie sheet (see pic below). Spoon the pie filling (to taste, start with a spoonful or two) into the center of each square. Top with a square of roughly the same size. Pinch the edges shut on all sides, tucking any rough edges in (using a fork to crimp helps). Pierce the top 3x with a fork to let steam escape (so they won't be as likely to leak). At this point they can be frozen unbaked and popped in the oven on a moment's notice. Just make sure to wrap them up airtight and separated with wax paper so they don't stick to each other. Repeat with the second half of the dough.
Bake in preheated oven for 15 minutes or until golden brown. They are best fresh, but can be reheated later with some success. Makes about a dozen 3-4" pastries.